The term note has two primary meanings: 1) a sign used in music to represent the relative duration and pitch of a sound; and 2) a pitched sound itself. Notes are the “atoms” of much Western music: discretizations of musical phenomena that facilitate performance, comprehension, and analysis (Nattiez 1990, p.81n9).

The term “note” can be used in both generic and specific senses: one might say either “the piece Happy Birthday to You begins with two notes having the same pitch,” or “the piece begins with two repetitions of the same note.” In the former case, one uses “note” to refer to a specific musical event; in the latter, one uses the term to refer to a class of events sharing the same pitch.

Contents [hide] 1 Note name 1.1 Note designation in accordance with octave name 2 Written notes 3 Note frequency (hertz) 4 History of note names 5 See also 6 Source 7 External links

[edit] Note name

Two notes with fundamental frequencies in a ratio of any power of two (e.g. half, twice, or four times) will sound very similar. Because of that all notes with these kinds of relations can be grouped under the same pitch class. In traditional music theory pitch classes are represented by the first seven letters of the Latin alphabet (A, B, C, D, E, F and G) and various modifications added to these letters (more on this below). The span of notes between one pitch and another that is twice (or half) its frequency is called an octave. In order to differentiate two notes that have the same pitch class but fall into different octaves, the system of scientific pitch notation combines a letter name with an Arabic numeral designating a specific octave. For example, the now-standard tuning pitch for most Western music, 440 Hz, is named a′ or A4. There are two formal ways to define each note and octave, the Helmholtz system and the Scientific pitch notation.

Letter names are modified by the accidentals sharp (♯, similar to the symbol #) and flat (♭, similar to the letter b). These symbols respectively raise or lower a pitch by a semitone or half-step, which in modern tuning will multiply or divide (respectively) the frequency of the original note by , approximately 1.059. They are written after the note name: so, for example, F♯ represents F-sharp, B♭ is B-flat. Other accidentals, such as double-sharps and double-flats (which will raise or lower the frequency by two semitones), are also possible in traditional music theory: avoiding sharps/flats in the key signature, “C♯♯” yields D, when D’s sharp is in the signature. Assuming enharmonicity, it is possible that use of accidentals will create equivalences between pitches that are written differently. For instance, raising the note B to B♯ is equal to the note C. Assuming the elimination of all such equivalences, however, the complete chromatic scale adds five additional pitch classes to the original seven lettered notes for a total of 12, each separated by a half-step.

Notes that belong to the diatonic scale relevant in the context are sometimes called diatonic notes; notes that do not meet that criterion are then sometimes called chromatic notes.

In musical notation, alterations to the seven lettered pitches in the scale are indicated by placing an accidental immediately before the note symbol, or by use of a key signature. The natural symbol (♮), can be inserted before a note to cancel a previously indicated flat or sharp (so as “F♮” an F-sharp would become simply F).

Another style of notation, rarely used in English, uses the suffix “is” to indicate a sharp and “es” (only “s” after A and E) for a flat, e.g. Fis for F♯, Ges for G♭, Es for E♭. This system first arose in Germany and is used in almost all European countries whose main language is not English or a Romance language.

In most countries using this system, the letter H is used to represent what is B natural in English, the letter B represents the B♭, and Heses represents the B♭♭ (not Bes, which would also have fit into the system). Belgium and the Netherlands use the same suffixes, but applied throughout to the notes A to G, so that B♭ is Bes. Denmark also uses h, but uses bes instead of heses for B♭♭.[1]

This is a complete chart of a chromatic scale built on the note C4, or “middle C”:

Name	prime		second		third	fourth		fifth		sixth		seventh
Natural (English)	C		D		E	F		G		A		B
Sharp (symbol)		C♯		D♯			F♯		G♯		A♯
Flat (symbol)		D♭		E♭			G♭		A♭		B♭
Sharp (English name)		C sharp		D sharp			F sharp		G sharp		A sharp
Flat (English name)		D flat		E flat			G flat		A flat		B flat
Natural (Northern European)	C		D		E	F		G		A		H
Sharp (Northern European)		Cis		Dis			Fis		Gis		Ais
Flat (Northern European)		Des		Es			Ges		As		B
Variant (Flat & Natural) (BE, NL)	-	-	-	-	-	-	-	-	-	-	Bes	B
Southern European	Do		Re		Mi	Fa		Sol		La		Si
Variant names	Ut		-		-	-		So		-		Ti
Indian style	Sa		Re		Ga	Ma		Pa		Da		Ni
Korean style	Da		Ra		Ma	Ba		Sa		Ga		Na
Approx. Frequency [Hz]	262	277	294	311	330	349	370	392	415	440	466	494
MIDI note number	60	61	62	63	64	65	66	67	68	69	70	71

[edit] Note designation in accordance with octave name

The table of each octave and the frequencies for every note of pitch class A is shown below. The traditional (Helmholtz) system centers on the great octave (with capital letters) and small octave (with lower case letters). Lower octaves are named “contra” (with primes before), higher ones “lined” (with primes after). Another system (scientific) suffixes a number (starting with 0, or sometimes -1). In this system A4 is nowadays standardised to 440 Hz, lying in the octave containing notes from C4 (middle C) to B4. The lowest note on most pianos is A0, the highest C8. The MIDI system for electronic musical instruments and computers uses a straight count starting with note 0 for C-1 at 8.1758 Hz up to note 127 for G9 at 12,544 Hz.

Octave naming systems				frequency of A (Hz)
traditional	shorthand	numbered	MIDI nr
subsubcontra	Cˌˌˌ – Bˌˌˌ	C-1 – B-1	0 – 11	13.75
sub-contra	Cˌˌ – Bˌˌ	C0 – B0	12 – 23	27.5
contra	Cˌ – Bˌ	C1 – B1	24 – 35	55
great	C – B	C2 – B2	36 – 47	110
small	c – b	C3 – B3	48 – 59	220
one-lined	c′ – b′	C4 – B4	60 – 71	440
two-lined	c′′ – b′′	C5 – B5	72 – 83	880
three-lined	c′′′ – b′′′	C6 – B6	84 – 95	1760
four-lined	c′′′′ – b′′′′	C7 – B7	96 – 107	3520
five-lined	c′′′′′ – b′′′′′	C8 – B8	108 – 119	7040
six-lined	c′′′′′′ – b′′′′′′	C9 – B9	120 – 127	14080

[edit] Written notes

A written note can also have a note value, a code which determines the note’s relative duration. These note values include quarter notes (crotchets), eighth notes (quavers), and so on.

When notes are written out in a score, each note is assigned a specific vertical position on a staff position (a line or a space) on the staff, as determined by the clef. Each line or space is assigned a note name, these names are memorized by the musician and allows him or her to know at a glance the proper pitch to play on his or her instrument for each note-head marked on the page.

The staff above shows the notes C, D, E, F, G, A, B, C listen (help·info) and then in reverse order, with no key signature or accidentals.

[edit] Note frequency (hertz)

In all technicality, music can be composed of notes at any arbitrary frequency. Since the physical causes of music are vibrations of mechanical systems, they are often measured in hertz (Hz), with 1 Hz = 1 complete vibration per second. For historical and other reasons especially in Western music, only twelve notes of fixed frequencies are used. These fixed frequencies are mathematically related to each other, and are defined around the central note, A4. The current “standard pitch” or modern “concert pitch” for this note is 440 Hz, although this varies in actual practice (see History of pitch standards).

The note-naming convention specifies a letter, any accidentals (sharps/flats), and an octave number. Any note is an integer of half-steps away from middle A (A4). Let this distance be denoted n. If the note is above A4, then n is positive; if it is below A4, then n is negative. The frequency of the note (f) (assuming equal temperament) is then:

f = 2^n/12 × 440 Hz

For example, one can find the frequency of C5, the first C above A4. There are 3 half-steps between A4 and C5 (A4 → A♯4 → B4 → C5), and the note is above A4, so n = +3. The note’s frequency is:

f = 2^3/12 × 440 Hz ≈ 523.2511 Hz.

To find the frequency of a note below A4, the value of n is negative. For example, the F below A4 is F4. There are 4 half-steps (A4 → A♭4 → G4 → G♭4 → F4), and the note is below A4, so n = −4. The note’s frequency is:

f = 2^−4/12 × 440 Hz ≈ 349.2290 Hz.

Finally, it can be seen from this formula that octaves automatically yield factors of two times the original frequency, since n is therefore a multiple of 12 (12k, where k is the number of octaves up or down), and so the formula reduces to:

f = 2^12k/12 × 440 Hz = 2^k × 440 Hz,

yielding a factor of 2. In fact, this is the means by which this formula is derived, combined with the notion of equally-spaced intervals.

The distance of an equally tempered semitone is divided into 100 cents. So 1200 cents are equal to one octave — a frequency ratio of 2:1. This means that a cent is precisely equal to the 1200th root of 2, which is approximately 1.0005777895

For use with the MIDI (Musical Instrument Digital Interface) standard, a frequency mapping is defined by:

For notes in an A440 equal temperament, this formula delivers the standard MIDI note number. Any other frequencies fill the space between the whole numbers evenly. This allows MIDI instruments to be tuned very accurately in any microtuning scale, including non-western traditional tunings.

[edit] History of note names

Music notation systems have used letters of the alphabet for centuries. The 6th century philosopher Boethius is known to have used the first fifteen letters of the alphabet to signify the notes of the two-octave range that was in use at the time. Though it is not known whether this was his devising or common usage at the time, this is nonetheless called Boethian notation.

Following this, the system of repeating letters A-G in each octave was introduced, these being written as minuscules for the second octave and double minuscules for the third. When the compass of used notes was extended down by one note, to a G, it was given the Greek G (Γ), gamma. (It is from this that the French word for scale, gamme is derived, and the English word gamut, from “Gamma-Ut”, the lowest note in Medieval music notation.)

The remaining five notes of the chromatic scale (the black keys on a piano keyboard) were added gradually; the first being B which was flattened in certain modes to avoid the dissonant augmented fourth interval. This change was not always shown in notation, but when written, B♭ (B-flat) was written as a Latin, round “b”, and B♮ (B-natural) a Gothic b. These evolved into the modern flat and natural symbols respectively. The sharp symbol arose from a barred b, called the “cancelled b”.

In parts of Europe, including Germany, Poland and Russia, the natural symbol transformed into the letter H: in German music notation, H is B♮ (B-natural) and B is B♭ (B-flat).

In Italian, Portuguese, Greek, French and Russian notation the notes of scales are given also in terms of Do – Re – Mi – Fa – Sol – La – Si rather than C – D – E – F – G – A – B. These names follow the original names reputedly given by Guido d’Arezzo, who had taken them from the first syllables of the first six musical phrases of a Gregorian Chant melody Ut queant laxis, which began on the appropriate scale degrees. These became the basis of the solfege system. “Do” later replaced the original “Ut” for ease of singing (most likely from the beginning of Dominus, Lord), though “Ut” is still used in some places. “Si” or “Ti” was added as the seventh degree (from Sancte Johannes, St. John, to which the hymn is dedicated).

Staff (music)

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Play Script – Text Romeo and Juliet Act I Romeo and Juliet     Script of Act I Romeo and Juliet  The play by William Shakespeare Introduction This section contains the script of Act I of Romeo and Juliet the play by William Shakespeare. The enduring works of William Shakespeare feature many famous and well loved characters. Make a note of any unusual words that you encounter whilst reading the script of Romeo and Juliet and check their definition in the Shakespeare Dictionary The script of Romeo and Juliet is extremely long. To reduce the time to load the script of the play, and for ease in accessing specific sections of the script, we have separated the text of Romeo and Juliet into Acts. Please click Romeo and Juliet Script to access further Acts. Script / Text of Act I Romeo and Juliet ACT I PROLOGUE Two households, both alike in dignity, In fair Verona, where we lay our scene, From ancient grudge break to new mutiny, Where civil blood makes civil hands unclean. From forth the fatal loins of these two foes A pair of star-cross’d lovers take their life; Whole misadventured piteous overthrows Do with their death bury their parents’ strife. The fearful passage of their death-mark’d love, And the continuance of their parents’ rage, Which, but their children’s end, nought could remove, Is now the two hours’ traffic of our stage; The which if you with patient ears attend, What here shall miss, our toil shall strive to mend. SCENE I. Verona. A public place. Enter SAMPSON and GREGORY, of the house of Capulet, armed with swords and bucklers  SAMPSON  Gregory, o’ my word, we’ll not carry coals. GREGORY  No, for then we should be colliers. SAMPSON  I mean, an we be in choler, we’ll draw. GREGORY  Ay, while you live, draw your neck out o’ the collar. SAMPSON  I strike quickly, being moved. GREGORY  But thou art not quickly moved to strike. SAMPSON  A dog of the house of Montague moves me. GREGORY  To move is to stir; and to be valiant is to stand: therefore, if thou art moved, thou runn’st away. SAMPSON  A dog of that house shall move me to stand: I will take the wall of any man or maid of Montague’s. GREGORY  That shows thee a weak slave; for the weakest goes to the wall. SAMPSON  True; and therefore women, being the weaker vessels, are ever thrust to the wall: therefore I will push Montague’s men from the wall, and thrust his maids to the wall. GREGORY  The quarrel is between our masters and us their men. SAMPSON  ‘Tis all one, I will show myself a tyrant: when I have fought with the men, I will be cruel with the maids, and cut off their heads. GREGORY  The heads of the maids? SAMPSON  Ay, the heads of the maids, or their maidenheads; take it in what sense thou wilt. GREGORY  They must take it in sense that feel it. SAMPSON  Me they shall feel while I am able to stand: and ’tis known I am a pretty piece of flesh. GREGORY  ‘Tis well thou art not fish; if thou hadst, thou hadst been poor John. Draw thy tool! here comes two of the house of the Montagues. SAMPSON  My naked weapon is out: quarrel, I will back thee. GREGORY  How! turn thy back and run? SAMPSON  Fear me not. GREGORY  No, marry; I fear thee! SAMPSON  Let us take the law of our sides; let them begin. GREGORY  I will frown as I pass by, and let them take it as they list. SAMPSON  Nay, as they dare. I will bite my thumb at them; which is a disgrace to them, if they bear it. Enter ABRAHAM and BALTHASAR ABRAHAM  Do you bite your thumb at us, sir? SAMPSON  I do bite my thumb, sir. ABRAHAM  Do you bite your thumb at us, sir? SAMPSON  [Aside to GREGORY] Is the law of our side, if I say ay? GREGORY  No. SAMPSON  No, sir, I do not bite my thumb at you, sir, but I bite my thumb, sir. GREGORY  Do you quarrel, sir? ABRAHAM  Quarrel sir! no, sir. SAMPSON  If you do, sir, I am for you: I serve as good a man as you. ABRAHAM  No better. SAMPSON  Well, sir. GREGORY  Say ‘better:’ here comes one of my master’s kinsmen. SAMPSON  Yes, better, sir. ABRAHAM  You lie. SAMPSON  Draw, if you be men. Gregory, remember thy swashing blow. They fight Enter BENVOLIO BENVOLIO  Part, fools! Put up your swords; you know not what you do. Beats down their swords Enter TYBALT TYBALT  What, art thou drawn among these heartless hinds? Turn thee, Benvolio, look upon thy death. BENVOLIO  I do but keep the peace: put up thy sword, Or manage it to part these men with me. TYBALT  What, drawn, and talk of peace! I hate the word, As I hate hell, all Montagues, and thee: Have at thee, coward! They fight Enter, several of both houses, who join the fray; then enter Citizens, with clubs First Citizen  Clubs, bills, and partisans! strike! beat them down! Down with the Capulets! down with the Montagues! Enter CAPULET in his gown, and LADY CAPULET CAPULET  What noise is this? Give me my long sword, ho! LADY CAPULET  A crutch, a crutch! why call you for a sword? CAPULET  My sword, I say! Old Montague is come, And flourishes his blade in spite of me. Enter MONTAGUE and LADY MONTAGUE MONTAGUE  Thou villain Capulet,–Hold me not, let me go. LADY MONTAGUE  Thou shalt not stir a foot to seek a foe. Enter PRINCE, with Attendants PRINCE  Rebellious subjects, enemies to peace, Profaners of this neighbour-stained steel,– Will they not hear? What, ho! you men, you beasts, That quench the fire of your pernicious rage With purple fountains issuing from your veins, On pain of torture, from those bloody hands Throw your mistemper’d weapons to the ground, And hear the sentence of your moved prince. Three civil brawls, bred of an airy word, By thee, old Capulet, and Montague, Have thrice disturb’d the quiet of our streets, And made Verona’s ancient citizens Cast by their grave beseeming ornaments, To wield old partisans, in hands as old, Canker’d with peace, to part your canker’d hate: If ever you disturb our streets again, Your lives shall pay the forfeit of the peace. For this time, all the rest depart away: You Capulet; shall go along with me: And, Montague, come you this afternoon, To know our further pleasure in this case, To old Free-town, our common judgment-place. Once more, on pain of death, all men depart. Exeunt all but MONTAGUE, LADY MONTAGUE, and BENVOLIO MONTAGUE  Who set this ancient quarrel new abroach? Speak, nephew, were you by when it began? BENVOLIO  Here were the servants of your adversary, And yours, close fighting ere I did approach: I drew to part them: in the instant came The fiery Tybalt, with his sword prepared, Which, as he breathed defiance to my ears, He swung about his head and cut the winds, Who nothing hurt withal hiss’d him in scorn: While we were interchanging thrusts and blows, Came more and more and fought on part and part, Till the prince came, who parted either part. LADY MONTAGUE  O, where is Romeo? saw you him to-day? Right glad I am he was not at this fray. BENVOLIO  Madam, an hour before the worshipp’d sun Peer’d forth the golden window of the east, A troubled mind drave me to walk abroad; Where, underneath the grove of sycamore That westward rooteth from the city’s side, So early walking did I see your son: Towards him I made, but he was ware of me And stole into the covert of the wood: I, measuring his affections by my own, That most are busied when they’re most alone, Pursued my humour not pursuing his, And gladly shunn’d who gladly fled from me. MONTAGUE  Many a morning hath he there been seen, With tears augmenting the fresh morning dew. Adding to clouds more clouds with his deep sighs; But all so soon as the all-cheering sun Should in the furthest east begin to draw The shady curtains from Aurora’s bed, Away from the light steals home my heavy son, And private in his chamber pens himself, Shuts up his windows, locks far daylight out And makes himself an artificial night: Black and portentous must this humour prove, Unless good counsel may the cause remove. BENVOLIO  My noble uncle, do you know the cause? MONTAGUE  I neither know it nor can learn of him. BENVOLIO  Have you importuned him by any means? MONTAGUE  Both by myself and many other friends: But he, his own affections’ counsellor, Is to himself–I will not say how true– But to himself so secret and so close, So far from sounding and discovery, As is the bud bit with an envious worm, Ere he can spread his sweet leaves to the air, Or dedicate his beauty to the sun. Could we but learn from whence his sorrows grow. We would as willingly give cure as know. Enter ROMEO BENVOLIO  See, where he comes: so please you, step aside; I’ll know his grievance, or be much denied. MONTAGUE  I would thou wert so happy by thy stay, To hear true shrift. Come, madam, let’s away. Exeunt MONTAGUE and LADY MONTAGUE BENVOLIO  Good-morrow, cousin. ROMEO  Is the day so young? BENVOLIO  But new struck nine. ROMEO  Ay me! sad hours seem long. Was that my father that went hence so fast? BENVOLIO  It was. What sadness lengthens Romeo’s hours? ROMEO  Not having that, which, having, makes them short. BENVOLIO  In love? ROMEO  Out– BENVOLIO  Of love? ROMEO  Out of her favour, where I am in love. BENVOLIO  Alas, that love, so gentle in his view, Should be so tyrannous and rough in proof! ROMEO  Alas, that love, whose view is muffled still, Should, without eyes, see pathways to his will! Where shall we dine? O me! What fray was here? Yet tell me not, for I have heard it all. Here’s much to do with hate, but more with love. Why, then, O brawling love! O loving hate! O any thing, of nothing first create! O heavy lightness! serious vanity! Mis-shapen chaos of well-seeming forms! Feather of lead, bright smoke, cold fire, sick health! Still-waking sleep, that is not what it is! This love feel I, that feel no love in this. Dost thou not laugh? BENVOLIO  No, coz, I rather weep. ROMEO  Good heart, at what? BENVOLIO  At thy good heart’s oppression. ROMEO  Why, such is love’s transgression. Griefs of mine own lie heavy in my breast, Which thou wilt propagate, to have it prest With more of thine: this love that thou hast shown Doth add more grief to too much of mine own. Love is a smoke raised with the fume of sighs; Being purged, a fire sparkling in lovers’ eyes; Being vex’d a sea nourish’d with lovers’ tears: What is it else? a madness most discreet, A choking gall and a preserving sweet. Farewell, my coz. BENVOLIO  Soft! I will go along; An if you leave me so, you do me wrong. ROMEO  Tut, I have lost myself; I am not here; This is not Romeo, he’s some other where. BENVOLIO  Tell me in sadness, who is that you love. ROMEO  What, shall I groan and tell thee? BENVOLIO  Groan! why, no. But sadly tell me who. ROMEO  Bid a sick man in sadness make his will: Ah, word ill urged to one that is so ill! In sadness, cousin, I do love a woman. BENVOLIO  I aim’d so near, when I supposed you loved. ROMEO  A right good mark-man! And she’s fair I love. BENVOLIO  A right fair mark, fair coz, is soonest hit. ROMEO  Well, in that hit you miss: she’ll not be hit With Cupid’s arrow; she hath Dian’s wit; And, in strong proof of chastity well arm’d, From love’s weak childish bow she lives unharm’d. She will not stay the siege of loving terms, Nor bide the encounter of assailing eyes, Nor ope her lap to saint-seducing gold: O, she is rich in beauty, only poor, That when she dies with beauty dies her store. BENVOLIO  Then she hath sworn that she will still live chaste? ROMEO  She hath, and in that sparing makes huge waste, For beauty starved with her severity Cuts beauty off from all posterity. She is too fair, too wise, wisely too fair, To merit bliss by making me despair: She hath forsworn to love, and in that vow Do I live dead that live to tell it now. BENVOLIO  Be ruled by me, forget to think of her. ROMEO  O, teach me how I should forget to think. BENVOLIO  By giving liberty unto thine eyes; Examine other beauties. ROMEO  ‘Tis the way To call hers exquisite, in question more: These happy masks that kiss fair ladies’ brows Being black put us in mind they hide the fair; He that is strucken blind cannot forget The precious treasure of his eyesight lost: Show me a mistress that is passing fair, What doth her beauty serve, but as a note Where I may read who pass’d that passing fair? Farewell: thou canst not teach me to forget. BENVOLIO  I’ll pay that doctrine, or else die in debt. Exeunt SCENE II. A street. Enter CAPULET, PARIS, and Servant  CAPULET  But Montague is bound as well as I, In penalty alike; and ’tis not hard, I think, For men so old as we to keep the peace. PARIS  Of honourable reckoning are you both; And pity ’tis you lived at odds so long. But now, my lord, what say you to my suit? CAPULET  But saying o’er what I have said before: My child is yet a stranger in the world; She hath not seen the change of fourteen years, Let two more summers wither in their pride, Ere we may think her ripe to be a bride. PARIS  Younger than she are happy mothers made. CAPULET  And too soon marr’d are those so early made. The earth hath swallow’d all my hopes but she, She is the hopeful lady of my earth: But woo her, gentle Paris, get her heart, My will to her consent is but a part; An she agree, within her scope of choice Lies my consent and fair according voice. This night I hold an old accustom’d feast, Whereto I have invited many a guest, Such as I love; and you, among the store, One more, most welcome, makes my number more. At my poor house look to behold this night Earth-treading stars that make dark heaven light: Such comfort as do lusty young men feel When well-apparell’d April on the heel Of limping winter treads, even such delight Among fresh female buds shall you this night Inherit at my house; hear all, all see, And like her most whose merit most shall be: Which on more view, of many mine being one May stand in number, though in reckoning none, Come, go with me. To Servant, giving a paper Go, sirrah, trudge about Through fair Verona; find those persons out Whose names are written there, and to them say, My house and welcome on their pleasure stay. Exeunt CAPULET and PARIS Servant  Find them out whose names are written here! It is written, that the shoemaker should meddle with his yard, and the tailor with his last, the fisher with his pencil, and the painter with his nets; but I am sent to find those persons whose names are here writ, and can never find what names the writing person hath here writ. I must to the learned.–In good time. Enter BENVOLIO and ROMEO BENVOLIO  Tut, man, one fire burns out another’s burning, One pain is lessen’d by another’s anguish; Turn giddy, and be holp by backward turning; One desperate grief cures with another’s languish: Take thou some new infection to thy eye, And the rank poison of the old will die. ROMEO  Your plaintain-leaf is excellent for that. BENVOLIO  For what, I pray thee? ROMEO  For your broken shin. BENVOLIO  Why, Romeo, art thou mad? ROMEO  Not mad, but bound more than a mad-man is; Shut up in prison, kept without my food, Whipp’d and tormented and–God-den, good fellow. Servant  God gi’ god-den. I pray, sir, can you read? ROMEO  Ay, mine own fortune in my misery. Servant  Perhaps you have learned it without book: but, I pray, can you read any thing you see? ROMEO  Ay, if I know the letters and the language. Servant  Ye say honestly: rest you merry! ROMEO  Stay, fellow; I can read. Reads ‘Signior Martino and his wife and daughters; County Anselme and his beauteous sisters; the lady widow of Vitravio; Signior Placentio and his lovely nieces; Mercutio and his brother Valentine; mine uncle Capulet, his wife and daughters; my fair niece Rosaline; Livia; Signior Valentio and his cousin Tybalt, Lucio and the lively Helena.’ A fair assembly: whither should they come? Servant  Up. ROMEO  Whither? Servant  To supper; to our house. ROMEO  Whose house? Servant  My master’s. ROMEO  Indeed, I should have ask’d you that before. Servant  Now I’ll tell you without asking: my master is the great rich Capulet; and if you be not of the house of Montagues, I pray, come and crush a cup of wine. Rest you merry! Exit BENVOLIO  At this same ancient feast of Capulet’s Sups the fair Rosaline whom thou so lovest, With all the admired beauties of Verona: Go thither; and, with unattainted eye, Compare her face with some that I shall show, And I will make thee think thy swan a crow. ROMEO  When the devout religion of mine eye Maintains such falsehood, then turn tears to fires; And these, who often drown’d could never die, Transparent heretics, be burnt for liars! One fairer than my love! the all-seeing sun Ne’er saw her match since first the world begun. BENVOLIO  Tut, you saw her fair, none else being by, Herself poised with herself in either eye: But in that crystal scales let there be weigh’d Your lady’s love against some other maid That I will show you shining at this feast, And she shall scant show well that now shows best. ROMEO  I’ll go along, no such sight to be shown, But to rejoice in splendor of mine own. Exeunt SCENE III. A room in Capulet’s house. Enter LADY CAPULET and Nurse  LADY CAPULET  Nurse, where’s my daughter? call her forth to me. Nurse  Now, by my maidenhead, at twelve year old, I bade her come. What, lamb! what, ladybird! God forbid! Where’s this girl? What, Juliet! Enter JULIET JULIET  How now! who calls? Nurse  Your mother. JULIET  Madam, I am here. What is your will? LADY CAPULET  This is the matter:–Nurse, give leave awhile, We must talk in secret:–nurse, come back again; I have remember’d me, thou’s hear our counsel. Thou know’st my daughter’s of a pretty age. Nurse  Faith, I can tell her age unto an hour. LADY CAPULET  She’s not fourteen. Nurse  I’ll lay fourteen of my teeth,– And yet, to my teeth be it spoken, I have but four– She is not fourteen. How long is it now To Lammas-tide? LADY CAPULET  A fortnight and odd days. Nurse  Even or odd, of all days in the year, Come Lammas-eve at night shall she be fourteen. Susan and she–God rest all Christian souls!– Were of an age: well, Susan is with God; She was too good for me: but, as I said, On Lammas-eve at night shall she be fourteen; That shall she, marry; I remember it well. ‘Tis since the earthquake now eleven years; And she was wean’d,–I never shall forget it,– Of all the days of the year, upon that day: For I had then laid wormwood to my dug, Sitting in the sun under the dove-house wall; My lord and you were then at Mantua:– Nay, I do bear a brain:–but, as I said, When it did taste the wormwood on the nipple Of my dug and felt it bitter, pretty fool, To see it tetchy and fall out with the dug! Shake quoth the dove-house: ’twas no need, I trow, To bid me trudge: And since that time it is eleven years; For then she could stand alone; nay, by the rood, She could have run and waddled all about; For even the day before, she broke her brow: And then my husband–God be with his soul! A’ was a merry man–took up the child: ‘Yea,’ quoth he, ‘dost thou fall upon thy face? Thou wilt fall backward when thou hast more wit; Wilt thou not, Jule?’ and, by my holidame, The pretty wretch left crying and said ‘Ay.’ To see, now, how a jest shall come about! I warrant, an I should live a thousand years, I never should forget it: ‘Wilt thou not, Jule?’ quoth he; And, pretty fool, it stinted and said ‘Ay.’ LADY CAPULET  Enough of this; I pray thee, hold thy peace. Nurse  Yes, madam: yet I cannot choose but laugh, To think it should leave crying and say ‘Ay.’ And yet, I warrant, it had upon its brow A bump as big as a young cockerel’s stone; A parlous knock; and it cried bitterly: ‘Yea,’ quoth my husband,’fall’st upon thy face? Thou wilt fall backward when thou comest to age; Wilt thou not, Jule?’ it stinted and said ‘Ay.’ JULIET  And stint thou too, I pray thee, nurse, say I. Nurse  Peace, I have done. God mark thee to his grace! Thou wast the prettiest babe that e’er I nursed: An I might live to see thee married once, I have my wish. LADY CAPULET  Marry, that ‘marry’ is the very theme I came to talk of. Tell me, daughter Juliet, How stands your disposition to be married? JULIET  It is an honour that I dream not of. Nurse  An honour! were not I thine only nurse, I would say thou hadst suck’d wisdom from thy teat. LADY CAPULET  Well, think of marriage now; younger than you, Here in Verona, ladies of esteem, Are made already mothers: by my count, I was your mother much upon these years That you are now a maid. Thus then in brief: The valiant Paris seeks you for his love. Nurse  A man, young lady! lady, such a man As all the world–why, he’s a man of wax. LADY CAPULET  Verona’s summer hath not such a flower. Nurse  Nay, he’s a flower; in faith, a very flower. LADY CAPULET  What say you? can you love the gentleman? This night you shall behold him at our feast; Read o’er the volume of young Paris’ face, And find delight writ there with beauty’s pen; Examine every married lineament, And see how one another lends content And what obscured in this fair volume lies Find written in the margent of his eyes. This precious book of love, this unbound lover, To beautify him, only lacks a cover: The fish lives in the sea, and ’tis much pride For fair without the fair within to hide: That book in many’s eyes doth share the glory, That in gold clasps locks in the golden story; So shall you share all that he doth possess, By having him, making yourself no less. Nurse  No less! nay, bigger; women grow by men. LADY CAPULET  Speak briefly, can you like of Paris’ love? JULIET  I’ll look to like, if looking liking move: But no more deep will I endart mine eye Than your consent gives strength to make it fly. Enter a Servant Servant  Madam, the guests are come, supper served up, you called, my young lady asked for, the nurse cursed in the pantry, and every thing in extremity. I must hence to wait; I beseech you, follow straight. LADY CAPULET  We follow thee. Exit Servant Juliet, the county stays. Nurse  Go, girl, seek happy nights to happy days. Exeunt SCENE IV. A street. Enter ROMEO, MERCUTIO, BENVOLIO, with five or six Maskers, Torch-bearers, and others  ROMEO  What, shall this speech be spoke for our excuse? Or shall we on without a apology? BENVOLIO  The date is out of such prolixity: We’ll have no Cupid hoodwink’d with a scarf, Bearing a Tartar’s painted bow of lath, Scaring the ladies like a crow-keeper; Nor no without-book prologue, faintly spoke After the prompter, for our entrance: But let them measure us by what they will; We’ll measure them a measure, and be gone. ROMEO  Give me a torch: I am not for this ambling; Being but heavy, I will bear the light. MERCUTIO  Nay, gentle Romeo, we must have you dance. ROMEO  Not I, believe me: you have dancing shoes With nimble soles: I have a soul of lead So stakes me to the ground I cannot move. MERCUTIO  You are a lover; borrow Cupid’s wings, And soar with them above a common bound. ROMEO  I am too sore enpierced with his shaft To soar with his light feathers, and so bound, I cannot bound a pitch above dull woe: Under love’s heavy burden do I sink. MERCUTIO  And, to sink in it, should you burden love; Too great oppression for a tender thing. ROMEO  Is love a tender thing? it is too rough, Too rude, too boisterous, and it pricks like thorn. MERCUTIO  If love be rough with you, be rough with love; Prick love for pricking, and you beat love down. Give me a case to put my visage in: A visor for a visor! what care I What curious eye doth quote deformities? Here are the beetle brows shall blush for me. BENVOLIO  Come, knock and enter; and no sooner in, But every man betake him to his legs. ROMEO  A torch for me: let wantons light of heart Tickle the senseless rushes with their heels, For I am proverb’d with a grandsire phrase; I’ll be a candle-holder, and look on. The game was ne’er so fair, and I am done. MERCUTIO  Tut, dun’s the mouse, the constable’s own word: If thou art dun, we’ll draw thee from the mire Of this sir-reverence love, wherein thou stick’st Up to the ears. Come, we burn daylight, ho! ROMEO  Nay, that’s not so. MERCUTIO  I mean, sir, in delay We waste our lights in vain, like lamps by day. Take our good meaning, for our judgment sits Five times in that ere once in our five wits. ROMEO  And we mean well in going to this mask; But ’tis no wit to go. MERCUTIO  Why, may one ask? ROMEO  I dream’d a dream to-night. MERCUTIO  And so did I. ROMEO  Well, what was yours? MERCUTIO  That dreamers often lie. ROMEO  In bed asleep, while they do dream things true. MERCUTIO  O, then, I see Queen Mab hath been with you. She is the fairies’ midwife, and she comes In shape no bigger than an agate-stone On the fore-finger of an alderman, Drawn with a team of little atomies Athwart men’s noses as they lie asleep; Her wagon-spokes made of long spiders’ legs, The cover of the wings of grasshoppers, The traces of the smallest spider’s web, The collars of the moonshine’s watery beams, Her whip of cricket’s bone, the lash of film, Her wagoner a small grey-coated gnat, Not so big as a round little worm Prick’d from the lazy finger of a maid; Her chariot is an empty hazel-nut Made by the joiner squirrel or old grub, Time out o’ mind the fairies’ coachmakers. And in this state she gallops night by night Through lovers’ brains, and then they dream of love; O’er courtiers’ knees, that dream on court’sies straight, O’er lawyers’ fingers, who straight dream on fees, O’er ladies ‘ lips, who straight on kisses dream, Which oft the angry Mab with blisters plagues, Because their breaths with sweetmeats tainted are: Sometime she gallops o’er a courtier’s nose, And then dreams he of smelling out a suit; And sometime comes she with a tithe-pig’s tail Tickling a parson’s nose as a’ lies asleep, Then dreams, he of another benefice: Sometime she driveth o’er a soldier’s neck, And then dreams he of cutting foreign throats, Of breaches, ambuscadoes, Spanish blades, Of healths five-fathom deep; and then anon Drums in his ear, at which he starts and wakes, And being thus frighted swears a prayer or two And sleeps again. This is that very Mab That plats the manes of horses in the night, And bakes the elflocks in foul sluttish hairs, Which once untangled, much misfortune bodes: This is the hag, when maids lie on their backs, That presses them and learns them first to bear, Making them women of good carriage: This is she– ROMEO  Peace, peace, Mercutio, peace! Thou talk’st of nothing. MERCUTIO  True, I talk of dreams, Which are the children of an idle brain, Begot of nothing but vain fantasy, Which is as thin of substance as the air And more inconstant than the wind, who wooes Even now the frozen bosom of the north, And, being anger’d, puffs away from thence, Turning his face to the dew-dropping south. BENVOLIO  This wind, you talk of, blows us from ourselves; Supper is done, and we shall come too late. ROMEO  I fear, too early: for my mind misgives Some consequence yet hanging in the stars Shall bitterly begin his fearful date With this night’s revels and expire the term Of a despised life closed in my breast By some vile forfeit of untimely death. But He, that hath the steerage of my course, Direct my sail! On, lusty gentlemen. BENVOLIO  Strike, drum. Exeunt SCENE V. A hall in Capulet’s house. Musicians waiting. Enter Servingmen with napkins  First Servant  Where’s Potpan, that he helps not to take away? He shift a trencher? he scrape a trencher! Second Servant  When good manners shall lie all in one or two men’s hands and they unwashed too, ’tis a foul thing. First Servant  Away with the joint-stools, remove the court-cupboard, look to the plate. Good thou, save me a piece of marchpane; and, as thou lovest me, let the porter let in Susan Grindstone and Nell. Antony, and Potpan! Second Servant  Ay, boy, ready. First Servant  You are looked for and called for, asked for and sought for, in the great chamber. Second Servant  We cannot be here and there too. Cheerly, boys; be brisk awhile, and the longer liver take all. Enter CAPULET, with JULIET and others of his house, meeting the Guests and Maskers CAPULET  Welcome, gentlemen! ladies that have their toes Unplagued with corns will have a bout with you. Ah ha, my mistresses! which of you all Will now deny to dance? she that makes dainty, She, I’ll swear, hath corns; am I come near ye now? Welcome, gentlemen! I have seen the day That I have worn a visor and could tell A whispering tale in a fair lady’s ear, Such as would please: ’tis gone, ’tis gone, ’tis gone: You are welcome, gentlemen! come, musicians, play. A hall, a hall! give room! and foot it, girls. Music plays, and they dance More light, you knaves; and turn the tables up, And quench the fire, the room is grown too hot. Ah, sirrah, this unlook’d-for sport comes well. Nay, sit, nay, sit, good cousin Capulet; For you and I are past our dancing days: How long is’t now since last yourself and I Were in a mask? Second Capulet  By’r lady, thirty years. CAPULET  What, man! ’tis not so much, ’tis not so much: ‘Tis since the nuptials of Lucentio, Come pentecost as quickly as it will, Some five and twenty years; and then we mask’d. Second Capulet  ‘Tis more, ’tis more, his son is elder, sir; His son is thirty. CAPULET  Will you tell me that? His son was but a ward two years ago. ROMEO  [To a Servingman] What lady is that, which doth enrich the hand Of yonder knight? Servant  I know not, sir. ROMEO  O, she doth teach the torches to burn bright! It seems she hangs upon the cheek of night Like a rich jewel in an Ethiope’s ear; Beauty too rich for use, for earth too dear! So shows a snowy dove trooping with crows, As yonder lady o’er her fellows shows. The measure done, I’ll watch her place of stand, And, touching hers, make blessed my rude hand. Did my heart love till now? forswear it, sight! For I ne’er saw true beauty till this night. TYBALT  This, by his voice, should be a Montague. Fetch me my rapier, boy. What dares the slave Come hither, cover’d with an antic face, To fleer and scorn at our solemnity? Now, by the stock and honour of my kin, To strike him dead, I hold it not a sin. CAPULET  Why, how now, kinsman! wherefore storm you so? TYBALT  Uncle, this is a Montague, our foe, A villain that is hither come in spite, To scorn at our solemnity this night. CAPULET  Young Romeo is it? TYBALT  ‘Tis he, that villain Romeo. CAPULET  Content thee, gentle coz, let him alone; He bears him like a portly gentleman; And, to say truth, Verona brags of him To be a virtuous and well-govern’d youth: I would not for the wealth of all the town Here in my house do him disparagement: Therefore be patient, take no note of him: It is my will, the which if thou respect, Show a fair presence and put off these frowns, And ill-beseeming semblance for a feast. TYBALT  It fits, when such a villain is a guest: I’ll not endure him. CAPULET  He shall be endured: What, goodman boy! I say, he shall: go to; Am I the master here, or you? go to. You’ll not endure him! God shall mend my soul! You’ll make a mutiny among my guests! You will set cock-a-hoop! you’ll be the man! TYBALT  Why, uncle, ’tis a shame. CAPULET  Go to, go to; You are a saucy boy: is’t so, indeed? This trick may chance to scathe you, I know what: You must contrary me! marry, ’tis time. Well said, my hearts! You are a princox; go: Be quiet, or–More light, more light! For shame! I’ll make you quiet. What, cheerly, my hearts! TYBALT  Patience perforce with wilful choler meeting Makes my flesh tremble in their different greeting. I will withdraw: but this intrusion shall Now seeming sweet convert to bitter gall. Exit ROMEO  [To JULIET] If I profane with my unworthiest hand This holy shrine, the gentle fine is this: My lips, two blushing pilgrims, ready stand To smooth that rough touch with a tender kiss. JULIET  Good pilgrim, you do wrong your hand too much, Which mannerly devotion shows in this; For saints have hands that pilgrims’ hands do touch, And palm to palm is holy palmers’ kiss. ROMEO  Have not saints lips, and holy palmers too? JULIET  Ay, pilgrim, lips that they must use in prayer. ROMEO  O, then, dear saint, let lips do what hands do; They pray, grant thou, lest faith turn to despair. JULIET  Saints do not move, though grant for prayers’ sake. ROMEO  Then move not, while my prayer’s effect I take. Thus from my lips, by yours, my sin is purged. JULIET  Then have my lips the sin that they have took. ROMEO  Sin from thy lips? O trespass sweetly urged! Give me my sin again. JULIET  You kiss by the book. Nurse  Madam, your mother craves a word with you. ROMEO  What is her mother? Nurse  Marry, bachelor, Her mother is the lady of the house, And a good lady, and a wise and virtuous I nursed her daughter, that you talk’d withal; I tell you, he that can lay hold of her Shall have the chinks. ROMEO  Is she a Capulet? O dear account! my life is my foe’s debt. BENVOLIO  Away, begone; the sport is at the best. ROMEO  Ay, so I fear; the more is my unrest. CAPULET  Nay, gentlemen, prepare not to be gone; We have a trifling foolish banquet towards. Is it e’en so? why, then, I thank you all I thank you, honest gentlemen; good night. More torches here! Come on then, let’s to bed. Ah, sirrah, by my fay, it waxes late: I’ll to my rest. Exeunt all but JULIET and Nurse JULIET  Come hither, nurse. What is yond gentleman? Nurse  The son and heir of old Tiberio. JULIET  What’s he that now is going out of door? Nurse  Marry, that, I think, be young Petrucio. JULIET  What’s he that follows there, that would not dance? Nurse  I know not. JULIET  Go ask his name: if he be married. My grave is like to be my wedding bed. Nurse  His name is Romeo, and a Montague; The only son of your great enemy. JULIET  My only love sprung from my only hate! Too early seen unknown, and known too late! Prodigious birth of love it is to me, That I must love a loathed enemy. Nurse  What’s this? what’s this? JULIET  A rhyme I learn’d even now Of one I danced withal. One calls within ‘Juliet.’ Nurse  Anon, anon! Come, let’s away; the strangers all are gone. Exeunt Script of Act I Romeo and Juliet by William Shakespeare

From Wikipedia, the free encyclopedia

This article focuses on modern scientific research on the origin of life. For alternate uses, see origin of life (disambiguation).

In the physical sciences, abiogenesis, the question of the origin of life, is the study of the nature in which life on Earth is theorized to have evolved from non-life sometime between 3.9 to 3.5 billion years ago. This topic also includes theories and ideas regarding possible extra-planetary or extra-terrestrial origin of life hypotheses, thought to have possibly occurred over the last 13.7 billion years in the evolution of the known universe since the big bang.

Origin of life studies is a limited field of research despite its profound impact on biology and human understanding of the natural world. Progress in this field is generally slow and sporadic, though it still draws the attention of many due to the eminence of the question being investigated. A few facts give insight into the conditions in which life may have emerged, but the mechanisms by which non-life became life are still elusive.

For the observed evolution of life on earth, see the timeline of life.

Contents [hide] 1 Current models 1.1 Origin of organic molecules 1.1.1 Miller’s experiments 1.1.2 Eigen’s hypothesis 1.1.3 Wächtershäuser’s hypothesis 1.2 From organic molecules to protocells 1.2.1 “Genes first” models: the RNA world 1.2.2 “Metabolism first” models: iron-sulfur world and others 1.2.3 Bubble Theory 1.2.4 Hybrid models 2 Other models 2.1 Autocatalysis 2.2 Clay theory 2.3 “Deep-hot biosphere” model of Gold 2.4 “Primitive” extraterrestrial life 2.5 The Lipid World 3 History of the concept 3.1 Aristotle 3.2 Pasteur 3.3 Darwin 3.4 Oparin 4 Relevant fields 5 See also 6 References 7 External links

[edit] Current models

There is no truly “standard” model of the origin of life. But most currently accepted models build in one way or another upon a number of discoveries about the origin of molecular and cellular components for life, which are listed in a rough order of postulated emergence:

1. Plausible pre-biotic conditions result in the creation of certain basic small molecules (monomers) of life, such as amino acids. This was demonstrated in the Miller-Urey experiment by Stanley L. Miller and Harold C. Urey in 1953.
2. Phospholipids (of an appropriate length) can spontaneously form lipid bilayers, a basic component of the cell membrane.
3. The polymerization of nucleotides into random RNA molecules might have resulted in self-replicating ribozymes (RNA world hypothesis).
4. Selection pressures for catalytic efficiency and diversity result in ribozymes which catalyse peptidyl transfer (hence formation of small proteins), since oligopeptides complex with RNA to form better catalysts. Thus the first ribosome is born, and protein synthesis becomes more prevalent.
5. Proteins outcompete ribozymes in catalytic ability, and therefore become the dominant biopolymer. Nucleic acids are restricted to predominantly genomic use.

The origin of the basic biomolecules, while not settled, is less controversial than the significance and order of steps 2 and 3. The basic inorganic chemicals from which life was thought to have formed are methane (CH₄), ammonia (NH₃), water (H₂O), hydrogen sulfide (H₂S), carbon dioxide (CO₂), and phosphate (PO₄^3-).

As of 2006, no one has yet synthesized a “protocell” using basic components which would have the necessary properties of life (the so-called “bottom-up-approach”). Without such a proof-of-principle, explanations have tended to be short on specifics. However, some researchers are working in this field, notably Steen Rasmussen at Los Alamos National Laboratory and Jack Szostak at Harvard University. Others have argued that a “top-down approach” is more feasible. One such approach, attempted by Craig Venter and others at The Institute for Genomic Research, involves engineering existing prokaryotic cells with progressively fewer genes, attempting to discern at which point the most minimal requirements for life were reached. The biologist John Desmond Bernal, coined the term Biopoesis for this process, and suggested that there were a number of clearly defined “stages” that could be recognised in explaining the origin of life.

Stage 1: The origin of biological monomers
Stage 2: The origin of biological polymers
Stage 3: The evolution from molecules to cell

Bernal suggested that Darwinian evolution may have commenced early, some time between Stage 1 and 2.

[edit] Origin of organic molecules

[edit] Miller’s experiments

Experiments were performed by Stanley Miller starting in 1953, under simulated conditions resembling those then thought to have existed shortly after Earth first accreted from the primordial solar nebula. The experiments are called the “Miller experiments“. The original experiment in 1953 was done by Miller as a graduate student and his professor Harold Urey. The experiment used a highly reduced mixture of gases (methane, ammonia and hydrogen). However, the composition of the prebiotic atmosphere of Earth is currently controversial. Other less reducing gases produce a lower yield and variety. It was once thought that appreciable amounts of molecular oxygen were present in the prebiotic atmosphere, which would have essentially prevented the formation of organic molecules; however, the current scientific consensus is that such was not the case.

The experiment showed that some of the basic organic monomers (such as amino acids) that form the polymeric building blocks of modern life can be formed spontaneously. Simple organic molecules are of course a long way from a fully functional self-replicating life form. But in an environment with no pre-existing life these molecules may have accumulated and provided a rich environment for chemical evolution (“soup theory“). On the other hand, the spontaneous formation of complex polymers from abiotically generated monomers under these conditions is not at all a straightforward process. Besides the necessary basic organic monomers, also compounds that would have prohibited the formation of polymers were formed in high concentration during the experiments.

Other sources of complex molecules have been postulated, including sources of extra-terrestrial stellar or interstellar origin. For example, from spectral analyses, organic molecules are known to be present in comets and meteorites. In 2004, a team detected traces of polycyclic aromatic hydrocarbons (PAH’s) in a nebula, the most complex molecule, to that date, found in space. The use of PAH’s has also been proposed as a precursor to the RNA world in the PAH world hypothesis.

It can be argued that the most crucial challenge unanswered by this theory is how the relatively simple organic building blocks polymerise and form more complex structures, interacting in consistent ways to form a protocell. For example, in an aqueous environment hydrolysis of oligomers/polymers into their constituent monomers would be favored over the condensation of individual monomers into polymers. Also, the Miller experiment produces many substances that would undergo cross-reactions with the amino acids or terminate the peptide chain.

[edit] Eigen’s hypothesis

In the early 1970s a major attack on the problem of the origin of life was organised by a team of scientists gathered around Manfred Eigen of the Max Planck Institute. They tried to examine the transient stages between the molecular chaos in a prebiotic soup and the transient stages of a self replicating hypercycle, between the molecular chaos in a prebiotic soup and simple macromolecular self-reproducing systems.

In a hypercycle, the information storing system (possibly RNA) produces an enzyme, which catalyzes the formation of another information system, in sequence until the product of the last aids in the formation of the first information system. Mathematically treated, hypercycles could create quasispecies, which through natural selection entered into a form of Darwinian evolution. A boost to hypercycle theory was the discovery that RNA, in certain circumstances forms itself into ribozymes, a form of RNA enzyme.

[edit] Wächtershäuser’s hypothesis

Another possible answer to this polymerization conundrum was provided in 1980s by Günter Wächtershäuser, in his iron-sulfur world theory. In this theory, he postulated the evolution of (bio)chemical pathways as fundamentals of the evolution of life. Moreover, he presented a consistent system of tracing today’s biochemistry back to ancestral reactions that provide alternative pathways to the synthesis of organic building blocks from simple gaseous compounds.

In contrast to the classical Miller experiments, which depend on external sources of energy (such as simulated lightning or UV irradiation), “Wächtershäuser systems” come with a built-in source of energy, sulfides of iron and other minerals (e.g. pyrite). The energy released from redox reactions of these metal sulfides is not only available for the synthesis of organic molecules, but also for the formation of oligomers and polymers. It is therefore hypothesized that such systems may be able to evolve into autocatalytic sets of self-replicating, metabolically active entities that would predate the life forms known today.

The experiment as performed, produced a relatively small yield of dipeptides (0.4% to 12.4%) and a smaller yield of tripeptides (0.003%) and the authors note that: “under these same conditions dipeptides hydrolysed rapidly.” Another criticism of the result is that the experiment did not include any organomolecules that would most likely cross-react or chain-terminate (Huber and Wächtershäuser, 1998).

The latest modification of the iron-sulfur-hypothesis was provided by William Martin and Michael Russell in 2002. According to their scenario, the first cellular life forms may have evolved inside so-called black smokers at seafloor spreading zones in the deep sea. These structures consist of microscale caverns that are coated by thin membraneous metal sulfide walls. Therefore, these structures would solve several critical points of the “pure” Wächtershäuser systems at once:

1. the micro-caverns provide a means of concentrating newly synthesised molecules, thereby increasing the chance of forming oligomers;
2. the steep temperature gradients inside a black smoker allow for establishing “optimum zones” of partial reactions in different regions of the black smoker (e.g. monomer synthesis in the hotter, oligomerisation in the colder parts);
3. the flow of hydrothermal water through the structure provides a constant source of building blocks and energy (freshly precipitated metal sulfides);
4. the model allows for a succession of different steps of cellular evolution (prebiotic chemistry, monomer and oligomer synthesis, peptide and protein synthesis, RNA world, ribonucleoprotein assembly and DNA world) in a single structure, facilitating exchange between all developmental stages;
5. synthesis of lipids as a means of “closing” the cells against the environment is not necessary, until basically all cellular functions are developed.

This model locates the “last universal common ancestor” (LUCA) inside a black smoker, rather than assuming the existence of a free-living form of LUCA. The last evolutionary step would be the synthesis of a lipid membrane that finally allows the organisms to leave the microcavern system of the black smokers and start their independent lives. This postulated late acquisition of lipids is consistent with the presence of completely different types of membrane lipids in archaebacteria and eubacteria (plus eukaryotes) with highly similar cellular physiology of all life forms in most other aspects.

Another unsolved issue in chemical evolution is the origin of homochirality, i.e. all monomers having the same “handedness” (amino acids being left handed, and nucleic acid sugars being right handed). Homochirality is essential for the formation of functional ribozymes (and probably proteins too). The origin of homochirality might simply be explained by an initial asymmetry by chance followed by common descent. Work performed in 2003 by scientists at Purdue identified the amino acid serine as being a probable root cause of organic molecules’ homochirality. Serine forms particularly strong bonds with amino acids of the same chirality, resulting in a cluster of eight molecules that must be all right-handed or left-handed. This property stands in contrast with other amino acids which are able to form weak bonds with amino acids of opposite chirality. Although the mystery of why left-handed serine became dominant is still unsolved, this result suggests an answer to the question of chiral transmission: how organic molecules of one chirality maintain dominance once asymmetry is established.

[edit] From organic molecules to protocells

The question “How do simple organic molecules form a protocell?” is largely unanswered but there are many hypotheses. Some of these postulate the early appearance of nucleic acids (“genes-first”) whereas others postulate the evolution of biochemical reactions and pathways first (“metabolism-first”). Recently, trends are emerging to create hybrid models that combine aspects of both.

[edit] “Genes first” models: the RNA world

Main article: RNA world hypothesis

The RNA world hypothesis suggests that relatively short RNA molecules could have spontaneously formed that were capable of catalyzing their own continuing replication. It is difficult to gauge the probability of this formation. A number of theories of modes of formation have been put forward. Early cell membranes could have formed spontaneously from proteinoids, protein-like molecules that are produced when amino acid solutions are heated – when present at the correct concentration in aqueous solution, these form microspheres which are observed to behave similarly to membrane-enclosed compartments. Other possibilities include systems of chemical reactions taking place within clay substrates or on the surface of pyrite rocks. Factors supportive of an important role for RNA in early life include its ability to replicate (see Spiegelman Monster); its ability to act both to store information and catalyse chemical reactions (as a ribozyme); its many important roles as an intermediate in the expression and maintenance of the genetic information (in the form of DNA) in modern organisms; and the ease of chemical synthesis of at least the components of the molecule under conditions approximating the early Earth.

A number of problems with the RNA world hypothesis remain, particularly the instability of RNA when exposed to ultraviolet light, the difficulty of activating and ligating nucleotides and the lack of available phosphate in solution required to constitute the backbone, and the instability of the base cytosine (which is prone to hydrolysis). Recent experiments also suggest that the original estimates of the size of an RNA molecule capable of self-replication were most probably vast underestimates. More-modern forms of the RNA World theory propose that a simpler molecule was capable of self-replication (that other “World” then evolved over time to produce the RNA World). At this time however, the various hypotheses have incomplete evidence supporting them. Many of them can be simulated and tested in the lab, but a lack of undisturbed sedimentary rock from that early in Earth’s history leaves few opportunities to test this hypothesis robustly.

[edit] “Metabolism first” models: iron-sulfur world and others

Several models reject the idea of the self-replication of a “naked-gene” and postulate the emergence of a primitive metabolism which could provide an environment for the later emergence of RNA replication.

One of the earliest incarnations of this idea was put forward in 1924 with Alexander Oparin‘s notion of primitive self-replicating vesicles which predated the discovery of the structure of DNA. More recent variants in the 1980s and 1990s include Günter Wächtershäuser‘s iron-sulfur world theory and models introduced by Christian de Duve based on the chemistry of thioesters. More abstract and theoretical arguments for the plausibility of the emergence of metabolism without the presence of genes include a mathematical model introduced by Freeman Dyson in the early 1980s and Stuart Kauffman‘s notion of collectively autocatalytic sets, discussed later in that decade.

However, the idea that a closed metabolic cycle, such as the reductive citric acid cycle, could form spontaneously (proposed by Günter Wächtershäuser) remains unsupported. According to Leslie Orgel, a leader in origin-of-life studies for the past several decades, there is reason to believe the assertion will remain so. In an article entitled “Self-Organizing Biochemical Cycles” (PNAS, vol. 97, no. 23, November 7, 2000, p12503-12507), Orgel summarizes his analysis of the proposal by stating, “There is at present no reason to expect that multistep cycles such as the reductive citric acid cycle will self-organize on the surface of FeS/FeS2 or some other mineral.” It is possible that another type of metabolic pathway was used at the beginning of life. For example, instead of the reductive citric acid cycle, the “open” acetyl-CoA pathway (another one of the four recognised ways of carbon dioxide fixation in nature today) would be even more compatible with the idea of self-organisation on a metal sulfide surface. The key enzyme of this pathway, carbon monoxide dehydrogenase/acetyl-CoA synthase harbours mixed nickel-iron-sulfur clusters in its reaction centers and catalyses the formation of acetyl-CoA (which may be regarded as a modern form of acetyl-thiol) in a single step.

[edit] Bubble Theory

Waves breaking on the shore create a delicate foam composed of bubbles. Winds sweeping across the ocean have a tendency to drive things to shore, much like driftwood collecting on the beach. It is possible that organic molecules were concentrated on the shorelines in much the same way. Shallow coastal waters also tend to be warmer, further concentrating the molecules through evaporation. While bubbles comprised of mostly water burst quickly, oily bubbles happen to be much more stable, lending more time to the particular bubble to perform these crucial experiments.

The phospholipid is a good example of an oily compound believed to have been prevalent in the prebiotic seas. Because phospholipids contain a hydrophilic head on one end, and a hydrophobic tail on the other, they have the tendency to spontaneously form lipid membranes in water. A lipid monolayer bubble can only contain oil, and is therefore not conducive to harbouring water-soluble organic molecules. On the other hand, a lipid bilayer bubble can contain water, and was a likely precursor to the modern cell membrane. If a protein came along that increased the integrity of its parent bubble, then that bubble had an advantage, and was placed at the top of the natural selection waiting list. Primitive reproduction can be envisioned when the bubbles burst, releasing the results of the experiment into the surrounding medium. Once enough of the ‘right stuff’ was released into the medium, the development of the first prokaryotes, eukaryotes, and multicellular organisms could be achieved. This theory is expanded upon in the book, “The Cell: Evolution of the First Organism” by Joseph Panno Ph.D.

Similarly, bubbles formed entirely out of protein-like molecules, called microspheres, will form spontaneously under the right conditions. But they are not a likely precursor to the modern cell membrane, as cell membranes are composed primarily of lipid compounds rather than amino-acid compounds.

[edit] Hybrid models

A growing realization of the inadequacy of either pure “genes-first” or “metabolism-first” models is leading the trend towards models that incorporate aspects of each.

[edit] Other models

[edit] Autocatalysis

British ethologist Richard Dawkins wrote about autocatalysis as a potential explanation for the origin of life in his 2004 book The Ancestor’s Tale. Autocatalysts are substances which catalyze the production of themselves, and therefore have the property of being a simple molecular replicator. In his book, Dawkins cites experiments performed by Julius Rebek and his colleagues at the Scripps Research Institute in California in which they combined amino adenosine and pentafluorophenyl ester with the autocatalyst amino adenosine triacid ester (AATE). One system from the experiment contained variants of AATE which catalysed the synthesis of themselves. This experiment demonstrated the possibility that autocatalysts could exhibit competition within a population of entities with heredity, which could be interpreted as a rudimentary form of natural selection.

[edit] Clay theory

A hypothesis for the origin of life based on clay was forwarded by Dr A. Graham Cairns-Smith of the University of Glasgow in 1985 and adopted as a plausible illustration by just a handful of other scientists (including Richard Dawkins). Clay theory postulates that complex organic molecules arose gradually on a pre-existing, non-organic replication platform — silicate crystals in solution. Complexity in companion molecules developed as a function of selection pressures on types of clay crystal is then exapted to serve the replication of organic molecules independently of their silicate “launch stage”. It is, truly, “life from a rock.”

Cairns-Smith is a staunch critic of other models of chemical evolution (see Genetic Takeover: And the Mineral Origins of Life ISBN 0-521-23312-7). However, he admits, that like many models of the origin of life, his own also has its shortcomings (Horgan 1991).

Peggy Rigou of the National Institute of Agronomic Research (INRA), in Jouy-en-Josas, France reports in the February 11, 2006 edition of Science News that prions are capable of binding to clay particles and migrate off the particles when the clay becomes negatively charged. While no reference is made in the report to implications for origin-of-life theories, this research may suggest prions as a likely pathway to early reproducing molecules.

[edit] “Deep-hot biosphere” model of Gold

The discovery of nanobes (filamental structures smaller than bacteria containing DNA) in deep rocks, led to a controversial theory put forward by Thomas Gold in the 1990s that life first developed not on the surface of the Earth, but several kilometers below the surface. It is now known that microbial life is plentiful up to five kilometers below the earth’s surface in the form of archaea, which are generally considered to have originated either before or around the same time as eubacteria, most of which live on the surface including the oceans. It is claimed that discovery of microbial life below the surface of another body in our solar system would lend significant credence to this theory. He also noted that a trickle of food from a deep, unreachable, source promotes survival because life arising in a puddle of organic material is likely to consume all of its food and become extinct.

[edit] “Primitive” extraterrestrial life

An alternative to Earthly abiogenesis is the hypothesis that primitive life may have originally formed extraterrestrially, either in space or on a nearby planet (Mars). (Note that exogenesis is related to, but not the same as, the notion of panspermia).

Organic compounds are relatively common in space, especially in the outer solar system where volatiles are not evaporated by solar heating. Comets are encrusted by outer layers of dark material, thought to be a tar-like substance composed of complex organic material formed from simple carbon compounds after reactions initiated mostly by irradiation by ultraviolet light. It is supposed that a rain of material from comets could have brought significant quantities of such complex organic molecules to Earth.

An alternative but related hypothesis, proposed to explain the presence of life on Earth so soon after the planet had cooled down, with apparently very little time for prebiotic evolution, is that life formed first on early Mars. Due to its smaller size Mars cooled before Earth(a difference of hundred of millions of years), allowing prebiotic processes there while Earth was still too hot. Life was then transported to the cooled Earth when crustal material was blasted off Mars by asteroid and comet impacts. Mars continued to cool faster and eventually became hostile to the continued evolution or even existence of life (it lost its atmosphere due to low volcanism), Earth is following the same fate as Mars, but at a slower rate.

Neither hypothesis actually answers the question of how life first originated, but merely shifts it to another planet or a comet. However, the advantage of an extraterrestrial origin of primitive life is that life is not required to have evolved on each planet it occurs on, but rather in a single location, and then spread about the galaxy to other star systems via cometary and/or meteorite impact. Evidence to support the plausibility of the concept is scant, but it finds support in recent study of Martian meteorites found in Antartica and in studies of extremophile microbes. ^[2] Additional support comes from a recent discovery of a bacterial ecosytem whose energy source is radioactivity.^[1]

[edit] The Lipid World

A theory that ascribes the first self-replicating object to be lipid-like.^[3]

 

From Wikipedia, the free encyclopedia

This article is about our solar system. For other planetary systems or star systems, see extrasolar planet.

The Solar System or solar system^[1] is the stellar system comprising the Sun and the retinue of celestial objects gravitationally bound to it: the eight planets, their 162 known moons^[2], three currently identified dwarf planets and their four known moons, and thousands of small bodies. This last category includes asteroids, meteoroids, comets, and interplanetary dust.

The principal component of the Solar System is the Sun (astronomical symbol ); a main sequence G2 star that contains 99.86% of the system’s known mass and dominates it gravitationally.^[3] Because of its large mass, the Sun has an interior density high enough to sustain nuclear fusion, releasing enormous amounts of energy, most of which is radiated into space in the form of electromagnetic radiation, including visible light. The Sun’s two largest orbiting bodies, Jupiter and Saturn, account for more than 90% of the system’s remaining mass. (The currently hypothetical Oort cloud, should its existence be confirmed, would also hold a substantial percentage).^[4]

In broad terms, the charted regions of the Solar System consist of the Sun, four rocky bodies close to it called the terrestrial planets, an inner belt of rocky asteroids, four gas giant planets, and an outer belt of small, icy bodies known as the Kuiper belt. In order of their distances from the Sun, the planets are Mercury ( ), Venus ( ), Earth ( ), Mars ( ), Jupiter ( ), Saturn ( ), Uranus ( ), and Neptune ( ). All planets but two are in turn orbited by natural satellites (usually termed “moons” after Earth’s Moon), and every planet past the asteroid belt is encircled by planetary rings of dust and other particles. The planets, with the exception of Earth, are named after gods and goddesses from Greco-Roman mythology.

From 1930 to 2006, Pluto ( ), one of the largest known Kuiper belt objects, was considered the Solar System’s ninth planet. However, in 2006 the International Astronomical Union (IAU) created an official definition of the term “planet”^[5]. Under this definition, Pluto is reclassified as a dwarf planet, and there are eight planets in the Solar System. In addition to Pluto, the IAU currently recognizes two other dwarf planets: Ceres ( ) , the largest asteroid, and Eris, which lies beyond the Kuiper belt in a region called the scattered disc. Of the known dwarf planets, only Ceres has no moons.

For many years, the Solar System was the only known example of planets in orbit around a star. The discovery in recent years of many extrasolar planets has led to the term “solar system” being applied generically to all stellar systems. Technically, however, it should strictly refer to Earth’s system only, as the word “solar” is derived from the Sun’s Latin name, Sol. Other stellar systems or planetary systems are usually referred to by the names of their parent star; “the Alpha Centauri system” or “the 51 Pegasi system”.

Contents [hide] 1 Layout 2 Planets, dwarf planets, and small solar system bodies 3 Formation 4 Sun 5 Inner planets 5.1 Mercury 5.2 Venus 5.3 Earth 5.4 Mars 6 Asteroid belt 6.1 Ceres 6.2 Asteroid groups 7 Outer planets 7.1 Jupiter 7.2 Saturn 7.3 Uranus 7.4 Neptune 8 Kuiper belt 8.1 Pluto and Charon 9 Comets 10 Scattered disc 10.1 Eris 11 Farthest regions 11.1 Sedna 11.2 Heliopause 12 Galactic context 13 Discovery and exploration 13.1 Telescopic observations 13.2 Observations by spacecraft 14 See also 15 References and notes 16 External links

Layout

Most objects in orbit round the Sun lie within the same shallow plane, called the ecliptic, which is roughly parallel to the Sun’s equator. The planets lie very close to the ecliptic, while comets and kuiper belt objects often lie at significant angles to it. All of the planets, and most other objects, also orbit with the Sun’s rotation in a counter-clockwise direction as viewed from a point above the Sun’s north pole. There is a direct relationship between how far away a planet is from the Sun, and how quickly it orbits. Mercury, with the smallest orbital circumference, travels the fastest, while Neptune, being much farther from the Sun, travels more slowly.

A planet’s distance from the Sun varies in the course of its year. Its closest approach to the Sun is known as its perihelion, while its farthest point from the Sun is called its aphelion. Though planets follow nearly circular orbits, with perihelions roughly equal to their aphelions, many comets, asteroids and objects of the Kuiper belt follow highly elliptical orbits, with large differences between perihelion and aphelion.

Astronomers most often measure distances within the solar system in astronomical units, or AU. One AU is the average distance between the Earth and the Sun, or roughly 149 598 000 km (93,000,000 mi). Pluto is roughly 38 AU from the Sun, while Jupiter lies at roughly 5.2 AU.

Informally, the Solar System is sometimes divided into separate “zones”; the first zone, known as the inner Solar System, comprises the inner planets and the main asteroid belt. The outer solar system is sometimes defined as everything beyond the asteroids; however, it is also the name often given to the region beyond Neptune, with the gas giants as a separate “middle zone.”

One common misconception with regards to the Solar System is that the orbits of the major objects (planets, Pluto, and asteroids) are equidistant. Due to the vast distances involved, many representations of the Solar System tend to simplify these orbits, with equal spacing between each object. However, with certain exceptions, it can generally be stated that the farther a planet or belt is from the Sun, the greater the distance between it and the previous orbit. For example, Venus is approximately 0.33 AU farther out than Mercury, whereas Jupiter lies 1.9 AU from the farthest extent of the asteroid belt, and Neptune’s orbit is roughly 20 AU farther out than that of Uranus. Attempts have been made to determine a correlation between these distances (see Bode’s Law) but to date there is no accepted theory that explains the respective orbital distances.

Planets, dwarf planets, and small solar system bodies

Main articles: Planet, Dwarf planet, and Small solar system body

Further information: 2006 redefinition of planet

A planet, according to the recent definition passed by the International Astronomical Union General Assembly on August 24, 2006, is any body in orbit around the Sun that a) has enough mass to form itself into a spherical shape and b) has cleared its immediate neighborhood of all smaller objects. Eight objects in the Solar System currently meet this definition; they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

Dwarf planet is a newly defined classification for stellar objects. The key difference between planets and dwarf planets is that while both are required to orbit the Sun and be of large enough mass that their own gravity pulls them into a nearly round shape, dwarf planets are not required to clear their neighborhood of other celestial bodies. Three objects in the solar system are currently included in this category; they are Pluto (formerly considered a planet), the asteroid Ceres, and the scattered disc object Eris. The IAU will begin evaluating other known objects to see if they fit within the definition of dwarf planets. The most likely candidates are some of the larger asteroids and several Trans-Neptunian Objects such as Sedna, Orcus, and Quaoar.

The remainder of the objects in the Solar System were classified as small solar system bodies. A small solar system body (SSSB) is a term defined in 2006 by the International Astronomical Union to describe Solar System objects which are neither planets nor dwarf planets.

All other objects … orbiting the Sun shall be referred to collectively as “Small Solar System Bodies” …. These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.^[6]

As of 2006, the IAU considers the following bodies to be SSSB’s:

1. all asteroids except Ceres
2. all centaurs
3. all trans-Neptunian objects, including Kuiper belt & Scattered disc objects, with the exception of Pluto and Eris
4. all comets

Formation

Main article: Formation and evolution of the solar system

The current hypothesis of Solar System formation is the nebular hypothesis, first proposed in 1755 by Immanuel Kant and independently formulated by Pierre-Simon Laplace.^[7] The nebular theory holds that the Solar System was formed from the gravitational collapse of a gaseous cloud called the solar nebula. It had a diameter of 100 AU and was 2–3 times the mass of the Sun. Over time, a disturbance (possibly a nearby supernova) squeezed the nebula, pushing matter inward until gravitational forces overcame the internal gas pressure and it began to collapse. As the nebula collapsed, conservation of angular momentum meant that it spun faster, and became warmer. As the competing forces associated with gravity, gas pressure, magnetic fields, and rotation acted on it, the contracting nebula began to flatten into a spinning protoplanetary disk with a gradually contracting protostar at the center.

From this cloud and its gas and dust, the various planets formed. The inner solar system was too warm for volatile molecules like water and methane to condense, and so the planetesimals which formed there were relatively small (comprising only 0.6% the mass of the disc) and composed largely of compounds with high melting points, such as silicates and metals. These rocky bodies eventually became the terrestrial planets. Farther out, the gravitational effects of Jupiter made it impossible for the protoplanetary objects present to come together, leaving behind the asteroid belt. Farther out still, beyond the frost line, Jupiter and Saturn developed as large gas giants, while Uranus and Neptune captured much less gas and are known as ice giants because their cores are believed to be made mostly of ice, that is, hydrogen compounds.

The gas giants were massive enough to retain a “primary atmosphere” of hydrogen and helium captured from the surrounding solar nebula. The terrestrial planets eventually lost their retained hydrogen and helium, and subsequently generated their own “secondary atmospheres” via volcanism, comet impacts, and, also in Earth’s case, the evolution of life.

After 100 million years, the pressure and density of hydrogen in the centre of the collapsing nebula became great enough for the protosun to begin thermonuclear fusion, which increased until hydrostatic equilibrium was achieved. The young Sun’s solar wind then cleared away all the gas and dust in the protoplanetary disk, blowing it into interstellar space, thus ending the growth of the planets.

Sun

Main article: Sun

The Sun is the Solar System’s parent star, and far and away its chief component. It is classed as a moderately large yellow dwarf. However, this name is misleading, as on the scale of stars in our galaxy, the Sun is rather large and bright. Stars are classified based on their position on the Hertzsprung-Russell diagram, a graph which plots the brightness of stars against their surface temperature. Generally speaking, the hotter a star is, the brighter it is. Stars which follow this pattern are said to be on the main sequence, and the Sun lies right in the middle of it. This has led many astronomy textbooks to label the Sun as “average;” however, stars brighter and hotter than it are rare, whereas stars dimmer and cooler than it are common. The vast majority of stars are dim red dwarfs, though they are under-represented in star catalogues as we can observe only those few that are very near the Sun in space.

The Sun’s position on the main sequence means, according to current theories of stellar evolution, that it is in the “prime of life” for a star, in that it has not yet exhausted its store of hydrogen for nuclear fusion, and been forced, as older red giants must, to fuse more inefficient elements such as helium and carbon. The Sun is growing increasingly bright as it ages. Early in its history, it was roughly 75 percent as bright as it is today.^[8]Calculations of the ratios of hydrogen and helium within the Sun suggest it is roughly halfway through its life cycle, and will eventually begin moving off the main sequence, becoming larger, brighter and redder, until, about five billion years from now, it too will become a red giant.

The Sun is a population I star, meaning that it is fairly new in galactic terms, having been born in the later stages of the universe’s evolution. As such, it contains far more elements heavier than hydrogen and helium (“metals” in astronomical parlance) than older population II stars such as those found in globular clusters. Since elements heavier than hydrogen and helium were formed in the cores of ancient and exploding stars, the first generation of stars had to die before the universe could be enriched with them. For this reason, the very oldest stars contain very little “metal”, while stars born later have more. This high “metallicity” is thought to have been crucial in the Sun’s developing a planetary system, because planets form from accretion of metals.^[9]

The Sun radiates a continuous stream of charged particles, a plasma known as solar wind, ejecting it outwards at speeds greater than 2 million kilometres per hour, creating a very tenuous “atmosphere” (the heliosphere), that permeates the solar system for at least 100 AU. This environment is known as the interplanetary medium. Small quantities of cosmic dust (some of it arguably interstellar in origin) are also present in the interplanetary medium and are responsible for the phenomenon of zodiacal light. The influence of the Sun’s rotating magnetic field on the interplanetary medium creates the largest structure in the solar system, the heliospheric current sheet.^[10]

Earth’s magnetic field protects its atmosphere from interacting with the solar wind. However, Venus and Mars do not have magnetic fields, and the solar wind causes their atmospheres to gradually bleed away into space.

Inner planets

Main article: Terrestrial planet

The four inner or terrestrial planets are characterised by their dense, rocky composition, few or no moons, and lack of ring systems. They are composed largely of minerals with high melting points such as silicates to form the planets’ solid crusts and semi-liquid mantles, and metallic dust grains such as iron, which forms their cores. Three of the four inner planets have atmospheres. All have impact craters, and all but one possess tectonic surface features, such as rift valleys and volcanoes. The term inner planet should not be confused with inferior planet, which designates those planets which are closer to the Sun than the Earth is (i.e. Mercury and Venus).

The four inner planets are:

Mercury

Mercury (0.4 AU), the closest planet to the Sun, is also the least massive of the planets, at only 0.055 Earth masses. Mercury has a very thin atmosphere consisting of atoms blasted off its surface by the solar wind. Because Mercury is so hot, these atoms quickly escape into space. Thus in contrast to the Earth and Venus whose atmospheres are stable, Mercury’s atmosphere is constantly being replenished.^[11] Mercury is surrounded by an extremely small amount of helium, hydrogen, oxygen, and sodium. This envelope of gases is so thin that the greatest possible atmospheric pressure (force exerted by the weight of gases) on Mercury would be about 0.000000000002 kg/cm² (0.00000000003 psi). The atmospheric pressure on the Earth is about 1.03 kg/cm² (14.7 psi).^[12] It has no natural satellite, and, to date, no observed geological activity save that produced by impacts. Its relatively large iron core and thin mantle have not yet been adequately explained. Hypotheses include that its outer layers were stripped off by a giant impact, and that it was prevented from fully accreting by the Sun’s gravity. The MESSENGER probe should aid in resolving this issue when it arrives in Mercury’s orbit in 2011.

Venus

Venus (0.7 AU), the first truly terrestrial planet, is of comparable mass to the Earth (0.815 Earth masses), and, like Earth, possesses a thick silicate mantle around an iron core, as well as a substantial atmosphere and evidence of one-time internal geological activity, such as volcanoes. However, it is much drier than Earth and its atmosphere is 90 times as dense and is composed overwhelmingly of carbon dioxide and sulfuric acid. Unlike Earth, evidence suggests that Venus’s crust is not divided into tectonic plates but instead comprises a single very thick rind.^[13] Venus has no natural satellite. It is the hottest planet, despite being farther from the sun than Mercury, with temperatures reaching more than 400 degrees Celsius. This is most likely due to the amount of greenhouse gases in the atmosphere.

Earth

The largest and densest of the inner planets, Earth (1 AU) is also the only one to demonstrate unequivocal evidence of current geological activity. Earth is the only planet known to have life. Its liquid hydrosphere, unique among the terrestrials, is probably the reason Earth is also the only planet where multi-plate tectonics has been observed, because water acts as a lubricant for subduction.^[14] Its atmosphere is radically different from the other terrestrials, having been altered by the presence of life to contain 21 percent free oxygen. Its satellite, the Moon, is sometimes considered a terrestrial planet in a co-orbit with its partner, because its orbit around the Sun never actually loops back on itself when observed from above.^[15] The Moon possesses many features in common with other terrestrial planets, though it lacks an iron core.

Mars

Mars (1.5 AU), at only 0.107 Earth masses, is less massive than either Earth or Venus. It possesses a tenuous atmosphere of carbon dioxide. Its surface, peppered with vast volcanoes and rift valleys such as Valles Marineris, shows that it was once geologically active and recent evidence^[16] suggests this may have been true until very recently. Mars possesses two tiny moons (Deimos and Phobos) thought to be captured asteroids.

Asteroid belt

Main article: Asteroid belt

Asteroids are mostly small solar system bodies that are composed in significant part of rocky, non-volatile minerals.

The main asteroid belt occupies the orbit between Mars and Jupiter, between 2.3 and 3.3 AU from the Sun. It is thought to be the remnants of a small terrestrial planet that failed to coalesce due to the gravitational interference of Jupiter. Asteroids range in size from hundreds of kilometers to as small as dust. All asteroids save the largest, Ceres, are classified as small solar system bodies; however, a number of other asteroids, such as Vesta and Hygeia, could potentially be reclassed as dwarf planets if it can be conclusively shown that they are spherical. The asteroid belt contains tens of thousands – and potentially millions – of objects over one kilometre in diameter.^[17] However, despite their large numbers, the total mass of the main belt is unlikely to be more than a thousandth of that of the Earth.^[18] In contrast to its various depictions in science fiction, the main belt is very sparsely populated; spacecraft routinely pass through without incident. Asteroids with a diameter of less than 50 m are called meteoroids.

Ceres

Ceres (2.77 AU) is the largest astronomical body in the asteroid belt and the only known dwarf planet in this region. It has a diameter of slightly under 1000 km, large enough for its own gravity to pull it into a spherical shape. Ceres was considered a planet when it was discovered in the nineteenth century, but was reclassified as an asteroid as further observation revealed additional asteroids. ^[19]

Asteroid groups

Asteroids in the main belt are subdivided into asteroid groups and families based on their specific orbital characteristics. Asteroid moons are asteroids that orbit larger asteroids. They are not as clearly distinguished as planetary moons, sometimes being almost as large as their partners. The asteroid belt also contains main-belt comets^[20] which may have been the source of Earth’s water.

Trojan asteroids are located in either of Jupiter’s L₄ or L₅ points, (gravitationally stable regions leading and trailing a planet in its orbit) though the term is also sometimes used for asteroids in any other planetary Lagrange point as well.

The inner solar system is also dusted with rogue asteroids, many of which cross the orbits of the inner planets.

Outer planets

Main article: Gas giant

The four outer planets, or gas giants, (sometimes called Jovian planets) are so large they collectively make up 99 percent of the mass known to orbit the Sun. Jupiter and Saturn are true giants, at 318 and 95 Earth masses, respectively, and composed largely of hydrogen and helium. Uranus and Neptune are both substantially smaller, being only 14 and 17 Earth masses, respectively. Their atmospheres contain a smaller percentage of hydrogen and helium, and a higher percentage of “ices”, such as water, ammonia and methane. For this reason some astronomers suggested that they belong in their own category, “Uranian planets,” or “ice giants.” All four of the gas giants exhibit orbital debris rings, although only the ring system of Saturn is easily observable from Earth. The term outer planet should not be confused with superior planet, which designates those planets which lie outside Earth‘s orbit (thus consisting of the outer planets plus Mars).

Jupiter

Jupiter (5.2 AU), at 318 Earth masses, is 2.5 times the mass of all the other planets put together. Its composition of largely hydrogen and helium is not very different from that of the Sun. Jupiter’s strong internal heat creates a number of semi-permanent features in its atmosphere, such as cloud bands and the Great Red Spot. Three of its 63 satellites, Ganymede, Io, and Europa share elements in common with the terrestrial planets, such as volcanism and internal heating. Ganymede has a larger diameter than Mercury.

Saturn

Saturn (9.5 AU), famous for its extensive ring system, has many qualities in common with Jupiter, including its atmospheric composition, though it is far less massive, being only 95 Earth masses. Two of its 49 moons, Titan and Enceladus, show signs of geological activity, though they are largely made of ice. Titan, like Ganymede, is larger than Mercury; it is also the only satellite in the solar system with a substantial atmosphere.

Uranus

Uranus (19.6 AU) at 14 Earth masses, is the lightest of the outer planets. Uniquely among the planets, it orbits the Sun on its side; its axial tilt lies at over ninety degrees to the ecliptic. Its core is remarkably cold (compared with the other gas giants; it is still several thousand degrees Celsius) and radiates very little heat into space. Uranus has 27 satellites, the largest being Titania, Oberon, Umbriel, Ariel and Miranda.

Neptune

Neptune (30 AU), though slightly smaller than Uranus, it is denser and slightly more massive, at 17 Earth masses, and radiates more internal heat than Uranus, but not as much as Jupiter or Saturn. Its peculiar ring system is composed of a number of dense “arcs” of material separated by gaps. Neptune has 13 moons. The largest, Triton, is geologically active, with geysers of liquid nitrogen.

Kuiper belt

The area beyond Neptune, often referred to as the outer solar system or simply the “trans-Neptunian region“, is still largely unexplored.

This region’s first formation, which actually begins inside the orbit of Neptune, is the Kuiper belt, a great ring of debris, similar to the asteroid belt but composed mainly of ice and far greater in extent, which lies between 30 and 50 AU from the Sun. This region is thought to be the place of origin for short-period comets, such as Halley’s comet. Though it is composed mainly of small solar system bodies, many of the largest Kuiper belt objects could soon be reclassified as dwarf planets. There are estimated to be over 100,000 Kuiper belt objects with a diameter greater than 50 km; however, the total mass of the Kuiper belt is relatively low, perhaps barely equalling the mass of the Earth.^[21] Many Kuiper belt objects have multiple satellites and most have orbits that take them outside the plane of the ecliptic.

The Kuiper belt can be roughly divided into two regions: the “resonant” belt, consisting of objects whose orbits are in some way linked to that of Neptune (orbiting, for instance, three times for every two Neptune orbits, or twice for every one), which actually begins within the orbit of Neptune itself, and the “classical” belt, consisting of objects that don’t have any resonance with Neptune, and which extends from roughly 39.4 AU to 47.7 AU.^[22]

Pluto and Charon

Pluto (39 AU average), is the largest known object in the Kuiper belt and was previously accepted as the smallest planet in the Solar System. It has now been reclassified as a dwarf planet by the Astronomers Congress organized by the International Astronomers Union (IAU). This decision was made on August 24, 2006.^[23] Pluto has a relatively eccentric orbit inclined 17 degrees to the ecliptic plane and ranging from 29.7 AU from the Sun at perihelion (within the orbit of Neptune) to 49.5 AU at aphelion. Prior to the 2006 redefinitions, Charon was considered a moon of Pluto, but in light of the redefinition it is unclear whether Charon will continue to be classified as a moon of Pluto or as a dwarf planet itself. Charon does not orbit Pluto, but rather both bodies orbit a barycenter of gravity in empty space, making Pluto-Charon a binary system. Two much smaller moons, Nix and Hydra, orbit Pluto and Charon.

Those Kuiper belt objects which, like Pluto, possess a 3:2 orbital resonance with Neptune (ie, they orbit twice for every three Neptunian orbits) are called Plutinos. Other Kuiper belt objects have different resonant orbits (2:1, 4:7, 3:5 etc) and are grouped accordingly. The remaining Kuiper belt objects, in more “classical” orbits, are classified as Cubewanos.

Comets

Comets are small solar system bodies (usually only a few kilometres across) composed largely of volatile ices, which possess highly eccentric orbits, generally having a perihelion within the orbit of the inner planets and an aphelion far beyond Pluto. When a comet approaches the Sun, its icy surface begins to sublimate, or boil away, creating a coma; a long tail of gas and dust which is often visible with the naked eye.

There are two basic types of comet: short-period comets, with orbits less than 200 years, and long-period comets, with orbits lasting thousands of years. Short-period comets are believed to originate in the Kuiper belt, while long period comets, such as Hale-Bopp (pictured), are believed to originate in the Oort Cloud. Some comets with hyperbolic orbits may originate outside the solar system. Old comets that have had most of their volatiles driven out by solar warming are often categorized as asteroids.

Centaurs are icy comet-like bodies that have less-eccentric orbits so that they remain in the region between Jupiter and Neptune. The first centaur to be discovered, 2060 Chiron, has been called a comet since it has been shown to develop a coma just as comets do when they approach the sun.^[24]

Scattered disc

Overlapping the Kuiper belt but extending much further outwards is the scattered disc. Scattered disc objects are believed to have been originally native to the Kuiper belt, but were ejected into erratic orbits in the outer fringes by the gravitational influence of Neptune’s outward migration (see Formation and evolution of the Solar System). Most scattered disc objects have perihelia within the Kuiper belt but aphelia as far as 150 AU from the Sun. Their orbits are also highly inclined to the ecliptic plane, and are often almost perpendicular to it. Some astronomers, such as Kuiper belt co-discoverer David Jewitt, consider the scattered disc to be merely another region of the Kuiper belt, and describe scattered disc objects as “scattered Kuiper belt objects.”^[25]

Eris

Eris (68 AU average) is the largest known scattered disc object and was the cause of the most recent debate about what constitutes a planet since it is at least 5% larger than Pluto with an estimated diameter of 2400 km (1500 mi). It is now the largest of the known dwarf planets.^[26] It has one moon, Dysnomia.

The object has many similarities with Pluto: its orbit is highly eccentric, with a perihelion of 38.2 AU (roughly Pluto’s distance from the Sun) and an aphelion of 97.6 AU, and is steeply inclined to the ecliptic plane, at 44 degrees, more so than any known object in the solar system except the newly-discovered object 2004 XR₁₉₀ (also known as “Buffy”^[27]) and is believed to consist largely of rock and ice.^[28]

Farthest regions

The point at which the solar system ends and interstellar space begins is not precisely defined, since its outer boundaries are delineated by two separate forces: the solar wind and the Sun‘s gravity. The solar wind extends to a point roughly 130 AU from the Sun, whereupon it surrenders to the surrounding environment of the interstellar medium. It is generally accepted, however, that the Sun’s gravity holds sway to the Oort cloud. This great mass of up to a trillion icy objects, currently hypothetical, is believed to be the source for all long-period comets and to surround the solar system like a shell from 50,000 to 100,000 AU beyond the Sun, or almost a quarter the distance to the next star system. The vast majority of the solar system, therefore, is completely unknown; however, recent observations of both the solar system and other star systems have led to an increased understanding of what is or may be lying at its outer edge.^[29]

Sedna

Sedna is a large, reddish Pluto-like object with a gigantic, highly elliptical orbit that takes it from about 76 AU at perihelion to 928 AU at aphelion and takes 12,050 years to complete. Mike Brown, who discovered the object in 2003, asserts that it cannot be part of the scattered disc or the Kuiper Belt as it has too distant a perihelion to have been affected by Neptune’s migration. He and other astronomers consider it to be the first in an entirely new population, one which also may include the object 2000 CR₁₀₅, which has a perihelion of 45 AU, an aphelion of 415 AU, and an orbital period of 3420 years. ^[30] Sedna is very likely a dwarf planet, though its shape has yet to be determined with certainty.

Heliopause

The heliosphere expands outward in a great bubble to about 95 AU, or three times the orbit of Pluto. The edge of this bubble is known as the termination shock; the point at which the solar wind collides with the opposing winds of the interstellar medium. Here the wind slows, condenses and becomes more turbulent, forming a great oval structure known as the heliosheath that looks and behaves very much like a comet’s tail; extending outward for a further 40 AU at its stellar-windward side, but tailing many times that distance in the opposite direction. The outer boundary of the sheath, the heliopause, is the point at which the solar wind finally terminates, and one enters the environment of interstellar space.^[31] Beyond the heliopause, at around 230 AU, lies the bow shock, a plasma “wake” left by the Sun as it travels through the Milky Way. ^[32]

Galactic context

The solar system is located in the Milky Way galaxy, a barred spiral galaxy with a diameter estimated at about 100,000 light years containing approximately 200 billion stars. Our Sun resides in one of the Milky Way’s outer spiral arms, known as the Orion Arm or Local Spur.^[33] The immediate galactic neighborhood of the solar system is known as the Local Fluff, an area of dense cloud in an otherwise sparse region known as the Local Bubble, an hourglass-shaped cavity in the interstellar medium roughly 300 light-years across. The bubble is suffused with high-temperature plasma that suggests it is the product of several recent supernovae.^[34]

Estimates place the solar system at between 25,000 and 28,000 light years from the galactic center. Its speed is about 220 kilometres per second, and it completes one revolution every 226 million years. The apex of solar motion–that is, the direction in which the Sun is heading–is near the current location of the bright star Vega.^[35] At the galactic location of the solar system, the escape velocity with regard to the gravity of the Milky Way is about 1000 km/s.

The solar system appears to have a very remarkable orbit. It is both extremely close to being circular, and at nearly the exact distance at which the orbital speed matches the speed of the compression waves that form the spiral arms. The solar system appears to have remained between spiral arms for most of the existence of life on Earth. The radiation from supernovae in spiral arms could theoretically sterilize planetary surfaces, preventing the formation of large animal life on land. By remaining out of the spiral arms, Earth may be unusually free to form large animal life on its surface. The solar system also lies well outside the star-crowded environs of the galactic centre. The opposing gravitational tugs from so many close stars within the galactic centre would have prevented planets from forming.^[36]

Recent studies of Extrasolar systems neighboring Earth’s have shown that our system’s configuration might not be common, as the vast majority so far discovered have been found to be markedly different. For instance, many extrasolar planetary systems contain a “hot Jupiter“^[37]; a planet of comparable size to Jupiter that nonetheless orbits very close to its star, at, for instance, 0.05 AU. It has been hypothesised that while the giant planets in these systems formed in the same place as the gas giants in Earth’s solar system did, some sort of migration took place which resulted in the giant planet spiralling in towards the parent star. Any terrestrial planets which had previously existed would presumably either be destroyed or ejected from the system. On the other hand, the apparent prevalence of hot Jupiters could result from a sampling error, as planets of similar size at greater distances from their stars are more difficult to detect.

Discovery and exploration

Main articles: Geocentric model, Heliocentrism, and Space exploration

For many thousands of years, people, with a few notable exceptions, did not believe the solar system existed. The Earth was believed not only to be stationary at the centre of the universe, but to be categorically different from the divine or ethereal objects that moved through the sky. The conceptual advances of the 17th century, led by Nicolaus Copernicus, Galileo Galilei, Johannes Kepler, and Isaac Newton, led gradually to the acceptance of the idea not only that Earth moved round the Sun, but that the planets were governed by the same laws that governed the Earth, and therefore could be similar to it.

Telescopic observations

The first exploration of the solar system was conducted by telescope, with astronomers learning that the Moon and other planets possessed such Earthlike features as craters, ice caps, and seasons.

Galileo Galilei was the first to notice the physical nature of our solar system. He discovered that the Moon was cratered, that the Sun was pocked with sunspots, and that Jupiter had four satellites in orbit around it. Giovanni Domenico Cassini and Christian Huygens followed on from Galileo’s discoveries by discovering the rings of Saturn, and Saturn’s moon Titan, respectively.

In 1682, Edmund Halley realised that repeated sightings of a comet were in fact recording the same object, returning regularly once every 75-6 years. This proved once and for all that comets were not atmospheric phenomena, as had been previously thought, and was the first evidence that anything other than the planets orbited the Sun.

In 1781, William Herschel was looking for binary stars in the constellation of Taurus when he observed what he thought was a new comet. In fact, its orbit revealed that it was a new planet, Uranus, the first ever discovered.

In 1801, Giuseppe Piazzi discoverd Ceres, a small world between Mars and Jupiter that was initially considered a new planet. However, subsequent discoveries of thousands of other small worlds in the same region led to their eventual separate reclassification: asteroids.

In 1846, discrepancies in the orbit of Uranus led many to suspect a large planet must be tugging at it from farther out. Urbain Le Verrier‘s calculations eventually led to the discovery of Neptune.

Further discrepancies in the orbits of the planets led Percival Lowell to conclude yet another planet, “Planet X” must still be out there. After his death, his Lowell Observatory conducted a search, which ultimately led to Clyde Tombaugh‘s discovery of Pluto in 1930. Pluto was, however, found to be too small to have disrupted the orbits of the outer planets, and its discovery was therefore coincidental. Like Ceres, it was initially considered to be a planet, but after the discovery of many other similarly sized objects in its vicinity it was eventually reclassified as a Kuiper belt object.

Observations by spacecraft

Since the start of the space age, a great deal of exploration has been performed by unmanned space missions that have been organized and executed by various space agencies. The first probe to land on another solar system body was the Soviet Union‘s Luna 2 probe, which impacted on the Moon in 1959. Since then, increasingly distant planets have been reached, with probes landing on Venus in 1965, Mars in 1976, the asteroid 433 Eros in 2001, and Saturn‘s moon Titan in 2005. Spacecraft have also made close approaches to other planets: Mariner 10 passed Mercury in 1973.

The first probe to explore the outer planets was Pioneer 10, which flew by Jupiter in 1973. Pioneer 11 was the first to visit Saturn, in 1979. The Voyager probes performed a grand tour of the outer planets following their launch in 1977, with both probes passing Jupiter in 1979 and Saturn in 1980 – 1981. Voyager 2 then went on to make close approaches to Uranus in 1986 and Neptune in 1989. The Voyager probes are now far beyond Neptune‘s orbit, and astronomers anticipate that they will encounter the heliopause which defines the outer edge of the solar system in the next few years. ^[38][39]

All planets in the solar system have now been visited to varying degrees by spacecraft launched from Earth, the last being Neptune in 1989. Through these unmanned missions, humans have been able to get close-up photographs of all of the planets and, in the case of landers, perform tests of the soils and atmospheres of some.

No Kuiper belt object has been visited by a man-made spacecraft. Launched in 19 January 2006, the New Horizons is currently enroute to becoming the first man-made spacecraft to explore this area. This unmanned mission is scheduled to fly by Pluto in July 2015. Should it prove feasible, the mission will then be extended to observe a number of other Kuiper belt objects.^[40]

See also

• List of solar system objects: By orbit—By mass—By radius
• Attributes of the largest solar system bodies
• Titius-Bode law
• Hypothetical planets
• Astronomical symbols
• Geological features of the solar system
• Numerical model of solar system
• Solar nebula
• Table of planetary attributes
• Timeline of discovery of solar system planets and their natural satellites
• Timeline of solar system astronomy
• Solar system model
• Space colonization
• Solar system in fiction
• Celestia – Space-simulation on your computer (OpenGL)

The Solar System  v·d·e
Sun	Mercury                Venus                Earth Moon                Mars Phobos, Deimos
Asteroid belt and minor planets	Vulcanoids | Main belt | Groups and families | Near-Earth asteroids | Jupiter Trojans See also Binary asteroids, Asteroid moons | The complete list of asteroids, and pronunciation of asteroid names.
Jupiter (moons | rings)	Amalthea group: Metis | Adrastea | Amalthea | Thebe; Galilean moons: Io | Europa | Ganymede | Callisto; Themisto*;         Himalia group: Leda | Himalia | Lysithea | Elara | S/2000 J 11;     Carpo*; S/2003 J 12*; Ananke group: [core] Ananke | Praxidike | Harpalyke | Iocaste | Euanthe | Thyone; [peripheral] Euporie | S/2003 J 3 | S/2003 J 18 | Thelxinoe | Helike | Orthosie | S/2003 J 16 | Hermippe | Mneme | S/2003 J 15; Carme group: S/2003 J 17 | S/2003 J 10 | Pasithee | Chaldene | Arche | Isonoe | Erinome | Kale | Aitne | Taygete | S/2003 J 9 | Carme | S/2003 J 5 | S/2003 J 19 | Kalyke | Eukelade | Kallichore; Pasiphaë group: Eurydome | S/2003 J 23 | Hegemone | Pasiphaë | Sponde | Cyllene | Megaclite | S/2003 J 4 | Callirrhoe | Sinope | Autonoe | Aoede | S/2003 J 14;         S/2003 J 2* (* not assigned to any group)
Saturn (moons | rings)	Shepherd moons: Pan | Daphnis | Atlas | Prometheus | Pandora; unverified shepherds: S/2004 S 6 | S/2004 S 4 | S/2004 S 3;             Epimetheus and Janus; Inner group: Mimas | Methone | Pallene | Enceladus | Telesto, Tethys, and Calypso | Polydeuces, Dione, and Helene;       Outer group: Rhea | Titan | Hyperion | Iapetus; Inuit group: Kiviuq | Ijiraq | S/2004 S 11 | Siarnaq | Paaliaq; Gallic group: Albiorix | Erriapo | Tarvos; Norse group: Phoebe | Skathi | S/2004 S 13 | Mundilfari | S/2004 S 17 | Narvi | S/2004 S 15 | S/2004 S 10 | Suttungr | S/2004 S 12 | S/2004 S 9 | S/2004 S 14 | S/2004 S 7 | Thrymr | S/2004 S 16 | Ymir | S/2004 S 8 | S/2004 S 18
Uranus (moons | rings)	Cordelia | Ophelia | Bianca | Cressida | Desdemona | Juliet | Portia | Rosalind | Cupid | Belinda | Perdita | Puck | Mab | Miranda | Ariel | Umbriel | Titania | Oberon | Francisco | Caliban | Stephano | Trinculo | Sycorax | Margaret | Prospero | Setebos | Ferdinand
Neptune (moons | rings)	Naiad | Thalassa | Despina | Galatea | Larissa | Proteus | Triton | Nereid | S/2002 N 1 | S/2002 N 2 | S/2002 N 3 | Psamathe | S/2002 N 4
Dwarf planets	Ceres | Pluto (moons : Charon · Nix · Hydra) | Eris (moon: Dysnomia)
Misc.	Centaurs | Damocloids | Comets (Lists of periodic and non-periodic comets) | Meteoroids Trans-Neptunians (Kuiper belt · Scattered disc · Oort cloud) | Heliosphere (Heliosheath · Heliopause · Hydrogen wall)
See also astronomical objects and the solar system‘s list of objects, sorted by radius or mass, and the pronunciation guide

From Wikipedia, the free encyclopedia

This article is about the astronomical object. For other uses, see Star (disambiguation).

A star is a massive, compact body of plasma in outer space that is held together by its own gravity and, unlike a planet, is sufficiently massive to sustain nuclear fusion in a very dense, hot core region. This fusion of atomic nuclei generates the energy that is continuously radiated from the outer layers of the star during much of its life span.^[1]

Individual stars differ in their total mass, composition, and age. The total mass of a star is the principal determinant in its evolution and eventual fate. A Hertzsprung-Russell diagram shows the pattern of the temperature of stars against their absolute magnitude, and can be used to determine the age of a star and the stage in its evolution. Initially, stars are composed primarily of hydrogen, with some helium and heavier trace elements that determine their metallicity. Over the course of a star’s evolution, a portion of the hydrogen is converted into helium and smaller quantities of heavier elements through the process of nuclear fusion. Part of the matter is then recycled into the interstellar environment and used to form a new generation of more metal-rich stars.^[2]

Binary and multi-star systems consist of two or more stars that are gravitationally bound, and generally move around each other in stable orbits. When two such stars have a relatively close orbit, their gravitational interaction can have a significant impact on their evolution.^[3] For example, a nova occurs when a white dwarf accretes matter from a companion star.

Contents [hide] 1 Observation history 1.1 Star designations 2 Units of measurement 3 Formation and evolution 3.1 Protostar formation 3.2 Main sequence 3.3 Post-main sequence 3.3.1 Massive stars 3.4 Collapse 4 Appearance and distribution 5 Age and size 6 Radiation 6.1 Luminosity 6.2 Magnitude 7 Classification 8 Variable stars 9 Structure 10 Nuclear fusion reaction pathways 11 References 12 See also 13 External links

Observation history

Stars have been important to every culture. They have been used in religious practices and for celestial navigation and orientation. The Gregorian calendar, used nearly everywhere in the world, is a solar calendar based on the position of the Earth relative to the nearest star, the Sun.

Early astronomers such as Tycho Brahe identified new stars in the heavens, suggesting that the heavens were not immutable. In 1584 Giordano Bruno suggested that the stars were actually other suns, and may have Earth-like planets in orbit around them.^[4] By the following century the idea of the stars as distant suns was reaching a consensus among astronomers, and it would be the theologian Richard Bentley who would prompt Isaac Newton to suggest that the stars were equally distributed in every direction, resulting in no net gravitational pull.^[5]

The Italian astronomer Geminiano Montanari recorded seeing variability in the star Algol 1667. Edmond Halley would then publish the first measurements of the proper motion of a pair of nearby “fixed” stars, demonstrating that they had changed position from the time of the ancient Greek astronomers Ptolemy and Hipparchus. But it would not be until 1838 that the first direct measurement of the distance to the star 61 Cygni was made by Friedrich Bessel using the parallax technique. Parallax measurements demonstrated the vast separation of the stars in the heavens.^[4]

Star designations

Main articles: star designation, astronomical naming conventions, and Star catalogue

The concept of the constellation arose as far back in history as Babylonian period. Ancient sky watchers imagined that prominent arrangements of stars formed patterns, and they associated these with particular aspects of nature or their myths. Twelve of these formations lay along the band of the ecliptic and these became the basis of astrology. Many of the more prominent individual stars were also given names, particularly with Arabic or Latin designations.

As well as certain constellations and the Sun itself, stars as a whole have their own myths.^[6] They were thought to be the souls of the dead or gods. An example of this is the star Algol, which was thought to represents the eye of the Gorgon Medusa.

To the Ancient Greeks, some “stars”, later identified as planets, represented various important deities, from which the names of the planets Mercury, Venus, Mars, Jupiter and Saturn were taken.^[6] (Uranus and Neptune were also Greek and Roman gods, but neither planet was known in Antiquity because of their low brightness. Their names were assigned by later astronomers.)

At about the year 1600, the names of the constellations were used to name the stars in the corresponding regions of the sky. The German astronomer Johann Bayer created a series of star maps and applied greek letters as designations to the stars in each constellation. Later the English astronomer John Flamsteed came up with a system using numbers, which would later be known as the Flamsteed designation. Numerous additional systems have since been created as star catalogues appeared.

The only body which has been recognized by the scientific community as having the authority to name stars or other celestial bodies is the International Astronomical Union (IAU).^[7] A number of private companies (for instance, the “International Star Registry“) purport to sell names to stars; however, these names are neither recognized by the scientific community nor used by them,^[7] and many in the astronomy community view these organizations as frauds preying on people ignorant of how stars are in fact named.^[8]

Units of measurement

Most stellar parameters are expressed in SI units by convention, but CGS units are also used (e.g., expressing luminosity in ergs per second). Mass, luminosity, and radii are usually given in solar units, based on the characteristics of the Sun:

solar mass:	kg[9]
solar luminosity:	watts[9]
solar radius:	m[10]

Large lengths, such as the radius of a giant star or the semi-major axis of a binary star system, are often expressed in terms of the astronomical unit (AU) — approximately the mean distance between the Earth and the Sun.

Formation and evolution

Main article: stellar evolution

Stars are formed within molecular clouds; large regions of high density in the interstellar medium (though still less dense than the inside of an earthly vacuum chamber). These clouds consist of mostly hydrogen with about 23–28% helium and a few percent heavier elements. One example of such a star-forming nebula is the Orion Nebula.^[11] As massive stars are formed from these clouds, they powerfully illuminate the clouds from which they formed, creating an H II region.

Protostar formation

Main article: star formation

The formation of a star begins with a gravitational instability inside a molecular cloud, often triggered by shockwaves from supernovae or the collision of two galaxies (as in a starburst galaxy). Once a region reaches a sufficient density of matter to satisfy the criteria for Jeans Instability it begins to collapse under its own gravitational force.

As the cloud collapses, individual Bok globules form with up to 50 solar masses of material. As a globule collapses and the density increases, the gravitational energy is converted into heat and the temperature rises. When the protostellar cloud has approximately reached hydrostatic equilibrium, a protostar forms at the core.^[12] These pre-main sequence stars are often surrounded by a protoplanetary disk. The protostar then follows a Hayashi track on the Hertzsprung-Russell diagram.^[13] The contraction will proceed until the Hayashi boundary is reached, and thereafter contraction will continue on a Kelvin-Helmholtz timescale with the temperature remaining stable. Stars with less than 0.5 solar masses thereafter join the main sequence. For more massive protostars, at the end of the Hayashi track they will slowly collapse in near hydrostatic equilibrium, following the Henyey track.^[14] The period of gravitational contraction lasts for about 10-15 million years.

Early stars of less than 2 solar masses are called T Tauri stars, while those with greater mass are Herbig Ae/Be stars. These newly-born stars emit jets of gas along their axis of rotation, producing small patches of nebulosity known as Herbig-Haro objects.^[15]

Main sequence

Main article: main sequence

Stars spend about 90% of their lifetime fusing hydrogen to produce helium in high-temperature and high-pressure reactions near the core. Such stars are said to be on the main sequence. Starting at zero age main sequence, the proportion of helium in a star’s core will steadily increase. As a consequence, in order to maintain the required rate of nuclear fusion at the core, the star will slowly increase in temperature and luminosity.^[16] The Sun, for example, is estimated to have increased in luminosity by about 40% since it reached the main sequence 4.6 billion years ago.^[17]

Every star generates a stellar wind of particles that causes a continual outflow of gas into space. For most stars, the amount of mass lost is negligible. The sun loses 10^-14 solar masses every year,^[18] or about 0.01% of its total mass over its entire lifespan. However very massive stars can lose 10^-7 to 10^-5 solar masses each year, significantly affecting their evolution.^[19] Stars that begin with more than 50 solar masses can lose over half their total mass while they remain on the main sequence.^[20]

The duration that a star spends on the main sequence depends primarily on the amount of fuel it has to burn and the rate at which it burns that fuel. In other words, its initial mass and its luminosity. For the Sun, this is estimated to be about 10¹⁰ years. However, the luminosity of a star is also determined by its mass. Consequently the total main sequence lifetime of a star can be estimated from its mass relative to the Sun’s as follows:^[21]

where M is the mass of the star and τ_ms is the star’s estimated main sequence lifetime in years.

Large stars burn their fuel very rapidly and are short-lived. Small stars (called red dwarfs) burn their fuel very slowly and last tens to hundreds of billions of years. At the end of their lives, they simply become dimmer and dimmer, fading into black dwarfs.^[22] However, since the lifespan of such stars is greater than the current age of the universe (13.7 billion years), no black dwarfs exist yet.

Besides mass, the portion of elements heavier than helium can play a significant role in the evolution of stars. This metallicity can influence the duration that a star will burn its fuel; control the formation of magnetic fields,^[23] and modifies the strength of the stellar wind.^[24] Older, population II stars have substantially less metallicity than the younger, population I stars due to the composition of the molecular clouds from which they formed. (Over time these clouds become increasingly enriched in heavier elements as older stars die and shed portions of their atmospheres.)

Post-main sequence

As most stars exhaust their supply of hydrogen at their core, their outer layers expand and cool to form a red giant. In about 5 billion years, when the Sun is a red giant, it will be so large that it will consume Mercury and Venus. Models predict that the Sun will expand out to about 99% of the distance to the Earth’s present orbit (1 astronomical unit, or AU). By that time, however, the orbit of the Earth will expand to about 1.7 AUs due to mass loss by the Sun and thus the Earth will escape envelopment.^[25]

In a red giant, hydrogen fusion proceeds in a shell-layer surrounding the core.^[26] Eventually the core is compressed enough to start helium fusion, and the star heats up and contracts. In low mass stars (less than the 1.4 solar mass) the helium fusion process begins with an explosive burst of energy generation known as a helium flash. The energy resulting from this event is equivalent to the luminosity of 10⁸ Suns, but it lasts only a few minutes. However, this energy goes into the elimination of the electron degeneracy at the core, and is not visible from the exterior.^[27]

Massive stars

Very high mass stars with more than nine solar masses can continue to fuse elements heavier than helium. The core contracts until the temperature and pressure are sufficient to fuse carbon. This process continues, with the successive stages being fueled by oxygen, neon, silicon, and sulfur. Near the end of the star’s life, fusion can occur along a series of onion-layer shells within the star. Each shell burns a different element, with the outermost shell burning hydrogen; the next shell burning helium, and so forth.^[28]

The final stage is reached when the star begins producing iron. Since iron nuclei are more tightly bound than any heavier nuclei, if they are fused they do not release energy — the process would, on the contrary, consume energy. Likewise, since they are more tightly bound than all lighter nuclei, energy cannot be released by fission.^[26] In relatively old, very massive stars, a large core of inert iron will accumulate in the center of the star. The heavier elements in these stars can work their way up to the surface, forming Wolf-Rayet stars with a dense stellar wind that sheds the outer atmosphere.

Collapse

An average-size star (less than 1.4 solar masses after explosion) will then shed its outer layers as a planetary nebula. The core that remains will be a tiny ball of electron degenerate matter not massive enough for further compression to take place, supported only by degeneracy pressure, called a white dwarf.^[29] These too will fade into brown, and then black dwarfs over a very long stretch of time. Electron degenerate matter is not plasma, even though stars are generally referred to as being spheres of plasma.

In larger stars, defined as having more than 1.4 solar masses after explosion, fusion continues until an iron core accumulates that is too large to be supported by electron degeneracy pressure. This core will suddenly collapse as its electrons are driven into its protons, forming neutrons and neutrinos in a burst of inverse beta decay, or electron capture. The shockwave formed by this sudden collapse causes the rest of the star to explode in a supernova. Supernovae are so bright that they may briefly outshine the star’s entire home galaxy. When they occur within the Milky Way, supernovae have historically been observed by naked-eye observers as “new stars” where none existed before.^[30]

Eventually, most of the matter in a star is blown away by the supernovae explosion (forming nebulae such as the Crab Nebula^[30]) and what remains will be a neutron star (sometimes a pulsar or X-ray burster) or, in the case of the largest stars (more than 3 solar masses after explosion), a black hole.^[31] In neutron stars and black holes, the star is not in a plasma state of matter, but either neutron degenerate matter or a state of matter not currently understood within the black hole.

The blown-off outer layers of dying stars include heavy elements which may be recycled during new star formation. These heavy elements allow the formation of rocky planets. The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium.^[30]

Appearance and distribution

Due to their great distance from the Earth, all stars except the Sun appear to the human eye as shining points in the night sky that twinkle because of the effect of the Earth’s atmosphere. The disks of stars are much too small in angular size to be observed with current ground-based optical telescopes, and so Interferometer telescopes are required in order to produce images of these objects. The Sun is also a star, but it is close enough to the Earth to appear as a disk instead, and to provide daylight. Other than the Sun, the star with the largest apparent size is R Doradus, with an angular diameter of only 0.057 arcseconds.^[32]

It has been a long-held assumption that the majority of stars occur in gravitationally-bound, multiple-star systems, forming binary stars. This is particularly true for very massive O and B class stars, where 80% of the systems are believed to be multiple. However the portion of single star systems increases for smaller stars, so that only 25% of red dwarfs are known to have stellar companions. As 85% of all stars are red dwarfs, most stars in the Milky Way are likely single from birth.^[33]

Larger groups called star clusters also exist. Stars are not spread uniformly across the universe, but are normally grouped into galaxies along with interstellar gas and dust. A typical galaxy contains hundreds of billions of stars, and there are more than 100 billion (10¹¹) galaxies in the observable universe.^[34]

Astronomers estimate that there are at least 70 sextillion (7×10²²) stars in the known universe.^[35] That is 230 billion times as many as the 300 billion in our own Milky Way.

The nearest star to the Earth, apart from the Sun, is Proxima Centauri, which is 39.9 trillion (10¹²) kilometres, or 4.2 light-years away. Light from Proxima Centauri takes 4.2 years to reach Earth. Travelling at the orbital speed of the Space Shuttle (5 miles per second—almost 30,000 kilometres per hour), it would take about 150,000 years to get there.^[36] Distances like this are typical inside galactic discs, where the solar system is located.^[37] Stars can be much closer to each other in the centres of galaxies and in globular clusters, or much farther apart in galactic halos.

Because of their low density, collisions of stars in the galaxy are thought to be rare. However in dense regions such as the core of stellar clusters or the galactic center, collisions can be more common.^[38] Such collisions can produce what are known as blue stragglers. These abnormal stars appear on a different part of the evolutionary track of the HR-diagram, effectively forming a merged star that has a higher surface temperature than the other main sequence stars in the cluster with the same luminosity. ^[39]

Small, dwarf stars such as the Sun generally have essentially featureless disks with only small starspots. Larger, giant stars have much bigger, much more obvious starspots,^[40] and also exhibit strong stellar limb darkening. That is, the brightness decreases towards the edge of the stellar disk.^[41] Red dwarf flare stars such as UV Ceti may also possess prominent starspot features.^[42]

Age and size

Almost everything about a star is determined by its initial mass, including its destiny and fate, as well as its essential characteristics, such as lifespan, luminosity, and size. Stars range in size from neutron stars no bigger than a city to supergiants like Betelgeuse in the Orion constellation, which has a diameter about 1,000 times larger than the Sun—about 1.6 billion kilometres. However, Betelgeuse has a much lower density than the Sun.^[43]

Many stars are between 1 billion and 10 billion years old. Some stars may even be close to 13.7 billion years old — the observed age of the universe.^[44] (See Big Bang theory and stellar evolution.) The more massive the star, the shorter its lifespan, primarily because massive stars have greater pressure on their cores, causing them to burn hydrogen more rapidly. The most massive stars last an average of about one million years, while stars of minimum mass (red dwarfs) burn their fuel very slowly and last tens to hundreds of billions of years.

Most of our understanding of stars comes from theoretical models and simulations based on spectral observations and measurements of the diameters of stars. The first measurement of the diameter of a star other than the Sun was made in 1921 by Albert Abraham Michelson on the Hooker telescope.^[45]

One of the most massive stars known is Eta Carinae,^[46] with 100–150 times as much mass as the Sun; its lifespan is very short — only several million years at most. A recent study of the Arches cluster suggests that 150 solar masses is the upper limit for stars in the current era of the universe.^[47] The reason for this limit is not precisely known, but is partially due to Eddington luminosity.

The first stars to form after the Big Bang may have been larger, up to 300 solar masses or more,^[48] due to the complete absence of elements heavier than lithium in their composition. This generation of supermassive, population III stars is long extinct, however, and currently only theoretical.

With a mass only 93 times that of Jupiter, AB Doradus C, a companion to AB Doradus A, is the smallest known star undergoing nuclear fusion in its core.^[49] For stars with similar metallicity to the Sun, the theoretical minimum mass the star can have, and still undergo fusion at the core, is estimated to be about 75 times the mass of Jupiter.^[50][51] When the metallicity is very low, however, a recent study of the faintest stars found that the minimum star size seems to be about 8.3% of the solar mass, or about 87 times the mass of Jupiter.^[52][51] Smaller bodies are called brown dwarfs, which occupy a poorly-defined grey area between stars and gas giants.

Radiation

The energy produced by stars, as a by-product of nuclear fusion, radiates into space as both electromagnetic radiation and particle radiation. The particle radiation emitted by a star is manifested as the stellar wind^[53] (which exists as a steady stream of electrically charged particles, such as free protons, alpha particles, and beta particles, emanating from the star’s outer layers) and as a steady stream of neutrinos emanating from the star’s core.

The production of energy at the core is the reason why stars shine so brightly: every time two or more atomic nuclei of one element fuse together to form an atomic nucleus of a new heavier element deep inside the core of a star, photons of electromagnetic energy are released from the nuclear fusion reaction, which are then converted to visible light in the star’s outer layers.

The peak frequency and color of the visible light depends on the temperature of the star’s outer layers, including its photosphere.^[54] Besides visible light, stars also emit forms of electromagnetic radiation that are invisible to the human eye. In fact, stellar electromagnetic radiation spans across the entire electromagnetic spectrum, from the longest wavelengths of radio waves and infrared to the shortest wavelengths of ultraviolet, X-rays, and gamma rays. All components of stellar electromagnetic radiation, both visible and invisible, are typically significant.

Using the stellar spectrum, astronomers can also determine the surface temperature, surface gravity, metallicity and rotation velocity of a star. If the distance of the star is known, such as by measuring the parallax, then the luminosity of the star can be derived. The mass, radius, surface gravity, and rotation period can then be estimated based on stellar models. (Mass can be measured directly for stars in binary systems. The technique of gravitational microlensing will also yield the mass of a star.^[55]) With these parameters, astronomers can also estimate the age of the star.^[56]

Luminosity

In astronomy, luminosity is the amount of light, and other forms of radiant energy, a star radiates per unit of time. The luminosity of a star can be approximated by treating the emitted energy as a black body radiation.^[57][58] So:

where L is the luminosity, σ is the Stefan-Boltzmann constant, R is the stellar radius and T is the effective temperature. This same formula can be used to compute the approximate radius of a main sequence star relative to the sun:

Magnitude

Main articles: apparent magnitude and absolute magnitude

The apparent brightness of a star is measured by its apparent magnitude, which is the brightness of a star with respect to the star’s luminosity, distance from Earth, and the altering of the star’s light as it passes through Earth’s atmosphere.

Number of stars brighter than magnitude

Apparent magnitude	Number  of Stars[59]
0	4
1	15
2	48
3	171
4	513
5	1,602
6	4,800
7	14,000

Intrinsic or absolute magnitude is what the apparent magnitude a star would be if the distance between the Earth and the star were 10 parsecs (32.6 light-years), and it is directly related to a star’s luminosity, measured from the standard distance of 10 parsecs.

Both the apparent and absolute magnitude scales are logarithmic units: one whole number difference in magnitude is equal to a brightness variation of about 2.5 times^[58] (the 5th root of 100 or 2.512 to be precise). This means that a first magnitude (+1.00) star is about 2.5 times brighter than a second magnitude (+2.00) star, and approximately 100 times brighter than a sixth magnitude (+6.00) star, which is the faintest star visible to the naked eye.

On both apparent and absolute magnitude scales, the smaller the magnitude number, the brighter the star; the larger the magnitude number, the fainter. The brightest stars, on either scale, have negative magnitude numbers. The variation in brightness between two stars is calculated by subtracting the magnitude number of the brighter star (m_b) from the magnitude number of the fainter star (m_f), then using the difference as an exponent for the base number 2.512; that is to say:

Δm = m_f − m_b

2.512^Δm = variation in brightness

Relative to both luminosity and distance from Earth, absolute magnitude (M) and apparent magnitude (m) are not exactly equivalent for an individual star;^[58] for example, the bright star Sirius has an apparent magnitude of -1.44, but it has an absolute magnitude of +1.41.

Our Sun has an apparent magnitude of -26.7, but its absolute magnitude is only +4.83. Sirius, the brightest star in the night sky, is approximately 23 times more luminous than our Sun, while Canopus, the second brightest star in the night sky, with an absolute magnitude of -5.53, is approximately 14,000 times more luminous than our Sun. Despite Canopus being vastly more luminous than Sirius, Sirius appears brighter than Canopus to our eyes, only because it is merely 8.6 light-years away from us, while Canopus is much further away from us at 310 light-years.

As of 2006, the star with the highest known absolute magnitude is LBV 1806-20, with a magnitude of -14.2. This star is 38,000,000 times more luminuous as our own sun.^[60] The least luminous stars that are currently known are located in the NGC 6397 cluster. There, the faintest red dwarf star was found, with a magnitude of 26, and a white dwarf of the 28th magnitude. These faint stars are so dim that their light is as bright as a birthday candle on the Moon when viewed from the Earth.^[61]

Classification

Main article: stellar classification

There are different classifications of stars according to their spectra ranging from type O, which are very hot, to M, which are so cool that molecules may form in their atmospheres. The main classifications in order of decreasing surface temperature are O, B, A, F, G, K, and M.

A variety of rare spectral types have special classifications. The most common of these are types L and T, which classify the coldest low-mass stars and brown dwarfs. Each letter has 10 subclassifications numbered (hottest to coldest) from 0 to 9. This system matches closely with temperature, but breaks down at the extreme hottest end; class O0 and O1 stars may not exist.^[62] (See Wolf-Rayet star.)

In addition, stars may be classified by their “luminosity effects”, which correspond to their spatial size. These range from 0 (hypergiants) through III (giants) to V (main sequence dwarfs) and VII (white dwarfs). Most stars fall into the main sequence which consists of ordinary hydrogen-burning stars. These fall along a narrow band when graphed according to their absolute magnitude and spectral type.^[62] Our Sun is a main sequence G2V (yellow dwarf), being of intermediate temperature and ordinary size.

Additional nomenclature, in the form of lower-case letters, can follow the spectral type to indicate peculiar features of the spectrum. For example, an “e” can indicate the presence of emission lines; “m” represents unusually strong levels of metals, and “var” can mean variations in the spectral type.^[62]

White dwarf stars, which typically fall in the lower left section of the Hertzsprung-Russell diagram, have their own class that begins with the letter D. This is further sub-divided into the classes DA, DB, DC, DO, DZ, and DQ, depending on the types of prominent lines found in the spectrum. This is followed by a numerical value that indicates the temperature index.^[63]

Variable stars

Main article: Variable star

Variable stars have periodic or random changes in luminosity because of intrinsic or extrinsic properties. Of the intrinsically variable stars, the primary types can be subdivided into three principal groups.

Pulsating variables are stars that vary in radius over time, expanding and contracting as a result of the stellar aging process. This category includes Cepheid and cepheid-like stars, and long-period variables such as Mira.^[64]

Eruptive variables are stars that experience sudden increases in luminosity because of flares or mass ejection events.^[64] This group includes protostars, Wolf-Rayet stars, and Flare stars, as well as giant and supergiant stars.

Cataclysmic or explosive variables undergo a dramatic change in their properties. This group includes novae and supernovae. A binary star system that includes a nearby white dwarf can produce certain types of these spectacular stellar explosions, including the nova and a Type 1a supernova.^[3] The explosion is created when the white dwarf accretes hydrogen from the companion star, building up mass until the hydrogen undergoes fusion.^[65] Some novae are also recurrent, having periodic outbursts of moderate amplitude.^[64]

Stars can also vary in luminosity because of extrinsic factors, such as eclipsing binaries, as well as rotating stars that produce extreme starspots.^[64] A notable example of an eclipsing binary is Algol, which regularly varies in magnitude from 2.3 to 3.5 over a period of 2.87 days.

Structure

Main article: Stellar structure

The interior of a stable, main sequence star is in a state of equilibrium in which the forces in any small volume exactly counterbalance each other. The balancing forces consist of inward directed gravitational force and the opposing pressure from the thermal energy of the plasma gas. For these forces to balance out, the temperature at the core of a typical star to be on the order of 10⁷ °C or higher. The resulting temperature and pressure at the hydrogen-burning core of a main sequence star are sufficient for nuclear fusion to occur, and for sufficient energy to be produced to prevent further collapse of the star.^[66]

As atomic nuclei are fused in the core, they emit energy in the form of gamma rays. These photons interact with the surrounding plasma, adding to the thermal energy at the core. Stars on the main sequence convert hydrogen into helium, creating a slowly but steadily increasing proportion of helium in the core. Eventually the helium content becomes predominant and energy production ceases at the core. Instead fusion occurs in a slowly expanding shell around the degenerate helium core.^[67]

In addition to hydrostatic equilibrium, the interior of a stable star will also maintain an energy balance of thermal equilibrium. There is a radial temperature gradient throughout the interior that results in a flux of energy flowing toward the exterior. The outgoing flux of energy leaving a shell within the star will exactly match the incoming flux.

The radiation zone is the region within the stellar interior where radiative transfer is sufficiently efficient to maintain the flux of energy. In this region the plasma will not be perturbed and any mass motions will die out. If this is not the case, however, then the plasma becomes unstable and convection will occur, forming a convection zone. This can occur, for example, in regions where very high energy fluxes occur, as near the core, or in areas with high opacity, as in the outer envelope.^[66]

The occurrence of convection in the outer envelope of a main sequence star depends on the spectral type. Massive stars several times the mass of the Sun have a convection zone deep within the interior and a radiative zone in the outer layers. Smaller stars such as the Sun are just the opposite, with the convective zone located in the outer layers.^[68] The convective zones will also vary over time as the star ages and the constitution of the interior is modified.^[66]

The portion of a main sequence star that is visible to an observer is called the photosphere. This is the layer at which the plasma gas of the star becomes transparent to photons of light. From here, the energy generated at the core becomes free to propagate out into space. It is within the photosphere that star spots, or regions of lower than average temperature, appear.

Above the level of the photosphere is the stellar atmosphere. In a main sequence star such as the Sun, the lowest level of the atmosphere is the thin chromosphere region, where spicules appear and stellar flares begin. This is surrounded by a transition region, where the temperature rapidly increases within a distance of only 100 km. Beyond this is the corona, a volume of super-heated plasma that can extend outward to several million kilometres.^[69] The existence of a corona appears to be dependent on a convective zone in the outer layers of the star.^[68] Despite its high temperature, the corona emits very little light. The corona region of the Sun is normally only visible during a solar eclipse.

From the corona, a stellar wind of plasma particles expands outward from the star, propagating until it interacts with the interstellar medium.

Nuclear fusion reaction pathways

Main article: Stellar nucleosynthesis

A variety of different nuclear fusion reactions take place inside the cores of stars, depending upon their mass and composition, as part of stellar nucleosynthesis. The net mass of the fused atomic nuclei is smaller than the sum of the constituents. This lost mass is converted into energy, according to the mass-energy relationship E=mc².^[1]

In the Sun, with a 10⁷ °K core, hydrogen fuses to form helium in the proton-proton chain reaction:^[70]

4¹H → 2²H + 2e⁺ + 2ν_e (4.0 MeV + 1.0 MeV)

2¹H + 2²H → 2³He + 2γ (5.5 MeV)

2³He → ⁴He + 2¹H (12.9 MeV)

These reactions result in the overall reaction:

4¹H → ⁴He + 2e⁺ + 2γ + 2ν_e (26.7 MeV)

In more massive stars, helium is produced in a cycle of reactions catalyzed by carbon, the carbon-nitrogen-oxygen cycle.^[70]

In stars with cores at 10⁸ K and masses between 0.5 and 10 solar masses, helium can be transformed into carbon in the triple-alpha process:^[70]

⁴He + ⁴He + 92 keV → ^8*Be

⁴He + ^8*Be + 67 keV → ^12*C

^12*C → ¹²C + γ + 7.4 MeV

For an overall reaction of:

3⁴He → ¹²C + γ + 7.2 MeV

In massive stars, heavier elements can also be burned in a contracting core through the Neon burning process and Oxygen burning process. The final stage in the stellar nucleosynthesis process is the Silicon burning process that results in the production of the stable isotope iron-56. Fusion can not proceed any further except through an endothermic process, and so further energy can only be produced through gravitational collapse.^[70]

The example below shows the amount of time required for a star of 20 solar masses to consume its nuclear fuel. As an O-class main sequence star, it would be 8 times the solar radius and 62,000 times the Sun’s luminosity.^[71]

Fuel material	Temperature (million Kelvin)	Density (kg/cm3)	Burn duration τ
H	37	0.0045	8.1 million years
He	188	0.97	1.2 million years
C	870	170	976 years
Ne	1,570	3,100	0.6 years
O	1,980	5,550	1.25 years
S/Si	3,340	33,400	11.5 days

From Wikipedia, the free encyclopedia

Contents [hide] 1 Disciplines 2 Physical cosmology 3 Metaphysical cosmology 4 Religious cosmology 5 Esoteric cosmology 6 References 6.1 Book references 7 External links

Cosmology, from the Greek: κοσμολογία (cosmologia, κόσμος (cosmos) order + λογια (logia) discourse) is the study of the Universe in its totality, and by extension, humanity’s place in it. Though the word cosmology is recent (first used in 1730 in Christian Wolff‘s Cosmologia Generalis), the study of the Universe has a long history involving science, philosophy, esotericism, and religion.

Disciplines

In recent times, physics and astrophysics have come to play a central role in shaping what is now known as physical cosmology, i.e. the understanding of the Universe through scientific observation and experiment. This discipline, which focuses on the Universe as it exists on the largest scales and at the earliest times, begins with the big bang, an expansion of space from which the Universe itself is said to have erupted ~13.7 ± 0.2 billion (10⁹) years ago. After its violent beginnings and until its very end, scientists then propose that the entire history of the Universe has been an orderly progression governed by physical laws.

In between the doctrines of religion and science, stands the philosophical perspective of metaphysical cosmology. This ancient field of study seeks to draw logical conclusions about the nature of the Universe, man, god and/or their connections based on the extension of some set of presumed facts borrowed from religion and/or observation.

Cosmology is often an important aspect of the origin beliefs of religions and mythologies that seek to explain the existence and nature of the reality. In some cases, views about the creation (cosmogony) and destruction (eschatology) of the Universe play a central role in shaping a framework of religious cosmology for understanding humanity’s role in the Universe.

Please expand this article. Further information might be found in a section of the talk page or at Requests for expansion.

A more contemporary distinction between religion and philosophy, esoteric cosmology is distinguished from religion in its less tradition-bound construction and reliance on modern “intellectual understanding” rather than faith, and from philosophy in its emphasis on spirituality as a formative concept.

Physical cosmology

Main article: Physical cosmology

Physical cosmology is the branch of physics and astrophysics that deals with the study of the physical origins of the Universe and the nature of the Universe on its very largest scales. In its earliest form it was what is now known as celestial mechanics, the study of the heavens. The Greek philosophers Aristarchus of Samos, Aristotle and Ptolemy proposed different cosmological theories. In particular, the geocentric Ptolemaic system was the accepted theory to explain the motion of the heavens until Nicolaus Copernicus, and subsequently Tycho Brahe, Johannes Kepler and Galileo Galilei proposed a heliocentric system in the 16th century. This is known as one of the most famous examples of epistemological rupture in physical cosmology.

With Isaac Newton and the 1687 publication of Principia Mathematica, the problem of the motion of the heavens was finally solved. Newton provided a physical mechanism for Kepler’s laws and his law of universal gravitation allowed the anomalies in previous systems, caused by gravitational interaction between the planets, to be resolved. A fundamental difference between Newton’s cosmology and those preceding it was the Copernican principle that the bodies on earth obey the same physical laws as all the celestial bodies. This was a crucial philosophical advance in physical cosmology.

Modern scientific cosmology is usually considered to have begun in 1917 with Albert Einstein‘s publication of his final modification of general relativity in the paper “Cosmological Considerations of the General Theory of Relativity,” (although this paper was not widely available outside of Germany until end of World War I). General relativity prompted cosmogonists such as Willem de Sitter, Karl Schwarzschild and Arthur Eddington to explore the astronomical consequences of the theory, which enhanced the growing ability of astronomers to study very distant objects. Prior to this (and for some time afterwards), physicists assumed that the Universe was static and unchanging. Subsequent modeling of the universe explored the possibility that the cosmological constant introduced by Einstein in that paper may result in an expanding universe, depending on its value. Thus the big bang theory was proposed by the Belgian priest Georges Lemaître in 1927 and rapidly confirmed by Edwin Hubble‘s discovery of the red shift in 1929 and later by the discovery of the cosmic microwave background radiation by Arno Penzias and Robert Woodrow Wilson in 1964.

Recent observations made by the COBE and WMAP satellites observing this background radiation have effectively, in many scientists eyes, transformed cosmology from a highly speculative science into a predictive science, as these observations matched predictions made by a theory called Cosmic inflation, which is a modification of the standard big bang theory. This has led many to refer to modern times as the “Golden age of cosmology”. ^[1]

Metaphysical cosmology

Main article: Cosmology (metaphysics)

In philosophy and metaphysics, cosmology deals with the world as the totality of space, time and all phenomena. Historically, it has had quite a broad scope, and in many cases was founded in religion. The ancient Greeks did not draw a distinction between this use and their model for the cosmos. However, in modern use it addresses questions about the Universe which are beyond the scope of science. It is distinguished from religious cosmology in that it approaches these questions using philosophical methods (e.g. dialectics). Modern metaphysical cosmology tries to address questions such as:

• What is the origin of the Universe? What is its first cause? Is its existence necessary? (see monism, pantheism, emanationism and creationism)
• What are the ultimate material components of the Universe? (see mechanism, dynamism, hylomorphism, atomism)
• What is the ultimate reason for the existence of the Universe? Does the cosmos have a purpose? (see teleology)

Religious cosmology

Main article: Religious cosmology

Many world religions have origin beliefs that explain the beginnings of the Universe and life. Often these are derived from scriptural teachings and held to be part of the faith’s dogma, but in some cases these are also extended through the use of philosophical and metaphysical arguments.

In some origin beliefs, the universe was created by a direct act of a god or gods who are also responsible for the creation of humanity (see creationism). In many cases, religious cosmologies also foretell the end of the Universe, either through another divine act or as part of the original design.

• Both Christianity and Judaism rely on the Genesis narrative as a scriptural account of cosmology. See also Biblical cosmology and Tzimtzum.
• Islam relies on understanding from the Qur’an as its major source for explaining cosmology. See Islamic cosmology
• Certain adherents of Buddhism, Hinduism (See also Hindu cosmology) and Jainism believe that the Universe passes through endless cycles of creation and destruction, each cycle lasting for trillions of years (e.g. 331 trillion years, or the life-span of Brahma, according to Hinduism), and each cycle with sub-cycles of local creation and destruction (e.g. 4.32 billion years, or a day of Brahma, according to Hinduism). The Vedic (Hindu) view of the world sees one true divine principle self-projecting as the divine word, ‘birthing’ the cosmos that we know from the monistic Hiranyagarbha or Golden Womb.
• A complex mixture of native Vedic gods, spirits, and demons, overlaid with imported Hindu and Buddhist deities, beliefs, and practices are the key to the Sri Lankan cosmology.

Many religions accept the findings of physical cosmology, in particular the big bang, and some, such as the Roman Catholic Church, have embraced it as suggesting a philosophical first cause. Others have tried to use the methodology of science to advocate for their own religious cosmology, as in intelligent design or creationist cosmologies.

Esoteric cosmology

Main article: Esoteric cosmology

Many esoteric and occult teachings involve highly elaborate cosmologies. These constitute a “map” of the Universe and of states of existences and consciousness according to the worldview of that particular doctrine. Such cosmologies cover many of the same concerns also addressed by religious and philosophical cosmology, such as the origin, purpose, and destiny of the Universe and of consciousness and the nature of existence. For this reason it is difficult to distinguish where religion or philosophy end and esotericism and/or occultism begins.

Common themes addressed in esoteric cosmology are emanation, involution, evolution, epigenesis, planes of existence, hierarchies of spiritual beings, cosmic cycles (e.g., cosmic year, Yuga), yogic or spiritual disciplines, and references to altered states of consciousness. Examples of esoteric cosmologies can be found in Gnosticism, The Urantia Book, Tantra (especially Kashmir Shaivism), Kabbalah, Sufism, Surat Shabda Yoga, Theosophy, Anthroposophy, the Fourth Way teaching of Gurdjieff and Ouspensky, the teachings of Patrizia Norelli-Bachelet, Gnostic circle and in The Rosicrucian Cosmo-Conception, as well as the book “God Speaks“ by Meher Baba.

The Mineral and Gemstone Kingdom

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	Chemical Formula	SiO2 · nH2O
	Composition	Hydrous silicon dioxide. The water can range from 3% to 21% of the total weight, but is usually between 6% to 10%.
	Color	Colorless, white, yellow, orange, red, purple, blue, green, gray, brown, and black. These are some of the base colors of Opal. Certain opals display different colors when viewed from different directions, or when the stone is turned, or when the light source is moved. This phenomenon, called play of color, gives a stone color flashes, or schillers of different colors which vary from stone to stone. The play of color in many Opals is truly exceptional and unsurpassed. Opals exhibiting such play of color are termed precious and are very valuable; those without it are termed common and have little or no value.
	Streak	White
	Hardness	Opal is listed in most references with a hardness of 5½. This is not fully correct, since some Opals are as low as 4½ and some as high as 6½.
	Crystal Forms and Aggregates	(Amorphous) Opal, being amorphous, is not really a mineral but a mineraloid. One of the scientifically accepted standards defining a mineral is that a mineral must have a crystal structure, which opal lacks. Many scientific groups and references, including the acclaimed Dana’s System of Mineralogy, categorize Opal together with the “true” minerals. For this reason, Opal — as well as other amorphous types that fall under the definition of a mineraloid — is mentioned in the mineral section of this guide. An organic form of Opal, known as Tabasheer or Pearl Opal, is formed in some species of bamboo. Opal occurs massive, botryoidal, reniform, stalactitic, earthy, nodular, as veins, in crusts, and in accumulating mounds. It often pseudomorphs after other minerals and organic matter, such as wood, shell, and bone.
	Transparency	Transparent to opaque
	Specific Gravity	Common Opal – 1.98 – 2.25 Precious Opal – 2.1 – 2.2
	Luster	Usually vitreous, but may also be pearly, waxy, or resinous
	Cleavage	None
	Fracture	Conchoidal
	Tenacity	Brittle
	Other ID Marks	1) Rich play of color in some specimens 2) Common Opal sometimes fluoresces, usually bright green, but also light green, light blue, purple, and white
	Varieties	There are many Opal varieties, each with its own name. Some are not scientifically recognized, but are universally used. The most important names are listed below; a much larger list can be found in the Opal variety page. (Opal variety page) Common Opal – Any Opal without play of color Precious Opal – Any Opal with play of color Black Opal – Precious Opal with a black, dark blue, dark green, dark gray or similar darkly colored background or base color. White Opal – Precious Opal with a light colored body color (white, yellow, cream, etc.) Fire Opal – Yellow-orange to red Opal Precious Fire Opal – Yellow-orange to red Opal with play of color Hyalite – Colorless, sky-blue, or misty-blue transparent variety of Opal.
	In Group	Silicates ; Tectosilicates ; Silica group May be classified as an oxide by a few abstract references (Oxide ; Hydroxide)
	All About	Many theories attempted to explain the cause of the play of color in Opal. In the 1960′s, the reason of the color play was discovered with the aid of the electron microscope. The following is a brief explanation: Opals are composed of tiny silica spheres that when arranged in an orderly pattern diffract the light entering the stone into the spectral colors. A light wave diffracted through the Opal causes a color sheen or scintillation in the stone. The density and pattern of the aligned silica spheres are responsible for the different colors refracted in the Opal. Common Opal lacks this effect, for its spheres are disordered or too compact to permit the light from refracting. The rich play of color in Precious Opal gives it unsurpassed splendor. For this reason, it is one of the most fascinating and fabled of gems. Specimens and jewels with a rich play of color command lofty prices. A condition called crazing affects certain opals, causing them to form internal and external cracks. Crazing is a particularly interesting phenomenon, for it lacks consistency and is unpredictable. Although it can occur at random, it usually strikes when an opal removed from damp conditions is allowed to dry too quickly, or when an opal is exposed to sudden intense light — or a combination of these factors. Crazing may also take place when an opal is subject to vibration, as during the cutting and polishing of a specimen. The severity of the crazing and the time it takes to “craze” varies among specimens. The origin of the specimen is often a determining factor to its resistance to crazing. A very gradual drying process over months or even years can in some cases effectively stabilize the stone and allow it to be cut and polished with a substantially reduced risk of crazing. Uncut Opals are often stored in water; this reduces the chance of crazing. Once a specimen is taken out of the water its susceptibility increases. Opal should not be taken out of the water for more than several minutes at a time. Cutting or polishing Opals, especially Opals from localities notorious for crazing, is a risky process; it is a matter of chance if the Opals will craze or not. To further protect Opals from crazing, they should not be washed with chemicals or detergents and should not be subject to sudden changes in temperature or lighting.
	Uses	Opal is one of the most precious gemstones. Black Opal is the most valuable and desired form, but White Opal and Precious Fire Opal are also quite costly. Opals are cut and polished into cabochons, and in a few rare cases are facetted into several cuts. Opal is also extremely popular among mineral collectors and museums compete to get the finest specimens. Common Opal (Opal without play of color) has no industrial or commercial use, except for those specimens that are brightly fluorescent and are collected by fluorescent mineral collectors. Also see the gemstone section on Opal
	Striking Features	Form, hardness, and opalescence
	Popularity (1-4)	1
	Prevalence (1-3)	1
	Demand (1-3)	1
	Distinguishing Similar Minerals	The hardness and forms of Opal distinguish it from all minerals. Some Opals resemble certain types of Chalcedony, but the hardness difference can distinguish the two.
	Commonly Occurs With	Chalcedony
	Noteworthy Localities	Common Opal (Opal without play of colors) is very common and occurs worldwide. It is beyond the scope of this guide to list all the significant Opal occurrences. Only important deposits of Precious Opal are mentioned here. Most Precious Opal is mined in Australia, the U.S., and Mexico. Some of the most famous Opal deposits are in Australia, and below are the most significant Australian localities: Andamooka, South Australia Coober Pedy, South Australia Lightning Ridge, New South Wales Mintabie, South Australia White Cliffs, New South Wales Since Queensland, Australia has numerous Opal producing areas in remote, deserted lands (sometimes hundreds of miles from the nearest community), only the names of the Opal fields are mentioned, instead of a town or village. Some of the most productive fields are Bull Creek, Hungerford, Opalton, Opalville, Quilpie and Yowah. In Mexico, Precious Opals and Fire Opals come from several deposits. The most important are near Queretaro, in Queretaro state, and near Magdalena, in Jalisco state. The U.S. has some of the most outstanding Opal occurrences. Virgin Valley, Humboldt Co., Nevada is rich in Opal mines producing all types of Precious Opal. Also worthy of mention are the Spencer area Opal mines in Clark Co., Idaho; Opal Butte, Morrow Co., Oregon; and the Last Chance Opal Mine, Kern Co., California. In Canada, a notable deposit exists in Vernon, British Columbia. Other significant worldwide Precious Opal deposits are in Ethiopia, the Czech Republic, Slovakia, Hungary, Turkey,  Indonesia, Brazil, Honduras, Guatemala, and Nicaragua.

 

sources

 

http://www.minerals.net/mineral/silicate/tecto/quartz/opal.htm 

 

 

Name:		Opal
Chem:		SiO2-n(H2O) hydrated silica
Crystal:		Isomorphic (no crystal structure)
Color:		yellow, clear, blue, gray, (all with or without color flash)
Refrac. Index:		1.44 – 1.46			Birefraction:		none
Hardness:		5.5 – 6.5			Spec. Grav.:		1.98-2.20
Fracture:		conchoidal			Cleavage:		none
Environment:		opal is a low temperature mineral and is found in cracks or cavities that are filled in late in their geological life. Water , must be present. It is also found as a replacement after certain skeletons of marine animals or plants.
Association:		often in porous substrates
Locals:		| Australia | Calif., Nev., Idaho, USA | Mexico | Brazil | Hungary |
Misc:		The name is probably from the Latin “opalus”, meaning “precious stone”. Opal can easily be dehydrated by heat or chemical exposure. Is very porous and can be damaged by many chemicals. Opal has been duplicated in the laboratory, and the material provides a very close approximation to the natural material. It is produced by the controlled (very slow) precipitation, and alignment of small silica spheres. The spheres form a three dimensional diffraction pattern which produces the color play. Beside synthetic opal there are also some opal “simulants” on the market. A simulant is something that resembles the natural material, but is composed or produced in an entirely different way. One such simulant is “Slocum Stone”, and appears to be a variety of glass. (See Synthetics for more man-made stones.)
Gem info:		Opal has a variety of poor gemstone characteristics, softness, dehydration, cracking, physical weakness, and sensitivity to heat. It also shows one of the best spectral displays of any gemstone, hence its value. It is made up of layers of precipitated silica spheres in a jelly-like water mass, and the ordering of the spheres sometimes produce a diffraction grating, that creates a play of rainbow sparkling light from within the stone. There are fundamentally three types of opal: precious opal (containing flashes of fire), the yellow-reddish “fire opal” which is named for its color (not flashes of fire), and common opal (sometimes called “potch”). “Common opal” is rarely transparent, but may be colored or contain inclusions. It is used as backing for the more desirable varieties of precious opal, but may also be cabbed to produce interesting stones. It comes in white, gray, yellow, blue, green, pink, and may be dendritic or contain moss. “fire opal” is named for its fiery red color, and not the flashes from within. Today most fire opal comes from Mexico and is often cut into faceted gem stones. It runs from a deep red to many shades of orange and even on to yellow. It may have a few flashes of fire, but usually it is sold for the color and clarity. It is not particularly expensive as it suffers from the same physical characteristics as all opal, and contains little of the desired color flash. “Precious Opal” – this is the material with the internal “color play”, “flash”, or “light show”. It is classified by its back ground color, the particular colors and intensity of color display, and its size. Stones that are predominantly white or light blue are the most common, and those that contain reds, oranges, and violets are considered more desirable. Blue and green are very common in most precious opal. Black opal, opal containing a predominantly dark background (dark-gray to blue-black) is the rarest, and most desired of all opals. When it contains reds and oranges it brings even a higher value. It may be priced right up with the top gemstones (diamond, emerald, and ruby). The very best black opal came from Lightening Ridge. Australia and small amounts till reach the market today, but there have been no major finds in many years. Another “collectors” variety is called “contra luz”. It shows the desired play of color, but only when light is transmitted through the stone. It appears to be clear when viewed from the same side. It is thus very difficult to design jewelry using this variety and it finds its way mainly into collections. “Hydrophane” is a variety that losses its water to become opaque, but can regain it’s water and become transparent with color flash, again mainly a collectors stone. Opal Doublets and Opal Triplets – these are sandwiched stones made up of 2 or more pieces. Further information is provided

 

 

 

Opal